Before your SS is allowed to come to stay with you, his father needs to address his behavior when he had a month long visit at your home.  He needs to explain that the behavior and initiative he displayed on his recent visit was not acceptable.   If he wants to take advantage of the opportunity provided, he needs to agree to some ground rules.

1.  Personal hygiene and cleanliness.  He needs to maintain himself and his space in an acceptable manner.  No one is expecting to be able to bounce a quarter off his bed each day, but he is expected to do his own laundry.. clean up after himself, make his bed.. and help with small chores around the house as needed.

2.  Civility is expected.  If he is sharing a home with other people he needs to acknowledge the other residents.. be polite.  If he truly is not in a mood to talk, he can go to his room but he should be able to say hello and answer a brief question of "how was your day?".  If he is experiencing social anxiety then I would insist that he attend counseling and get help with that.

3.  House Rules.  These will cover things like curfews (yeah.. you are over 18.. but you are living in my home and you won't be setting your own rules).  Time to rise and shine as well.  Unless he is working the night shift, there is no reason why he can't be up by 8 or 9 in the morning.  No unplanned visitors.. he has to get permission to bring people over.. again.. this is because he is living in your home.. not his own.  Respecting other people's belongings is expected as well.

4.  RENT.. YES.. he will have to pay it.  first month can be free until that paycheck.. but he needs to be paying you guys rent.  I would also include in what he has to pay you the cost of the vehicle insurance.. to make sure it gets paid..lol.  Now, if you guys want to put some or all of that rent away into an account to give back to him when it's time for him to move out?  that's absolutely fine.  But making him pay rent now will get him used to having to set that amount aside from  his monthly budget.. it may be too tempting to spend it otherwise.  His dad can help him set up an automatic savings from his paycheck too... but I would get that rent money so he learns to account for that.. and it's not a difficult transition when he does leave.  And if you have been saving it. .you can give him money that will cover deposits and setting up the new place!

4.  Your husband also needs to get his buy in that this IS what he wants.. he wants the new job.. he wants the fresh start.. not just he wants new scenery.

I would map out everything you expect from him and when you expect it by. I would add therapy and job coaching to the list. While going to school would be ideal, if he is that unmotivated, it'll just be a money suck. Perhaps working for peanuts will motivate him, or maybe he'll find a trade that he wants to pursue. I'd spend the first three months helping him get a job and getting established with a therapist and career counselor, then the next three months either helping him find schools to enroll into or help him come up with a long-term plan.

Also, make sure to cover that plan with him BEFORE he moves in and have him literally sign off on it. Let him decide before time, effort, and energy goes in to him if this is really what he wants.

Finally, make it clear to your DH that you WILL kick SS out at any point if he isn't holding up his end of the bargain. And if your DH won't make your SS leave if he isn't doing what is expected that you will. 

Good luck.

He wants a change of scenery? He can get that by staying with BM and getting a job there. Not sure why he has to move out of state to get it. I wouldn't do this. It will cause problems in your marriage when skid fails to do anything and your DH is reluctant to kick baby boy out.

If you want to help out SS I would draw these terms up as a contract and have him sign them before he moves in. I would also include exit clauses and what a breach of a rule will mean, ie: don't get a job and your out of here. 

Please make sure that your DH is on the same page with the ramifications, otherwise you might just end up with a year long version of the month you just went through.

Good luck to you - I hope it works out well and you get SS on the straight and narrow.

My SS21 lives with BM in a similar manner, and I'm pretty sure she would allow him to live with her for the rest of her life.  No way in HELL is he ever moving into my house.

Let well enough alone - he's grown now and can live how he pleases. Don't have him come live with you in hope that you can turn him around, you are inviting trouble. If he wants to improve his life, he'll do it on his own. 

Could not agree more with tog. Good intentions in step land, often (usually/always) end with living in step hell, with no way out. Except, maybe, divorce.

Six months to a a year, turns into 2 years, 5 years, with no change in behavior.

He came for a month and did NOTHING and was rude, yet, husband did nothing.

Do you really see this working out? I know it sounds good to "help him grow" but seriously, that should have been done a long time ago. He showed you who he is when he stayed for that month, your husband, too. He had a month to start teaching his son some basic stuff but did he?

He's 20, he really should be able to feed himself and make his bed. He's old enough to know basic manners. He's lazy and rude and he's not going to change for you or his dad.

If he wants a change in life, as opposed to a change of location to continue his slothfulness, he would make a change where he lives, now.

If that month was ANY indication of how SS will be, that would be a hard no from me.

SD22 Feral Forger lives at her mothers 1-bedroom apartment and sleeps on the couch. Shes was horrible in her teens and according to SD15 hasnt changed - in fact has gotten much worse. I have told DH that I will never ever ever live with her rude, disrespectful, mean, dirty, horrid self.

SD22 Feral Forger sounds much like you SS20. She has no job, and she has no drivers license and has not been taking college classes. She mooches. She is enabled. Last Christmas I invited her over and she just started arguing and yelling at me. Just so over it.

If SS WANTS to start becoming independant he should start NOW. I honestly think this is a mistake - Good Luck!

Personally, after his one month visit, it would be a hard no for me. Someone who wants a change and to be different would not have come to your home and did what he did for a month. But as you are determined for this to happen, you and your DH need to be VERY clear with him about expectations. Write it out, discuss in advance (if necessary, via Facetime or Skype), and make sure he understands that if he does not follow the rules, he heads back to BM. And then stand firm on the contract. 

DH had three similar to your SS. One came to visit and thought he was going to stay forever. Stayed out late in the club scene, slept all day, then woke up around 2 to go sell meat (yes, you read that right....sold meat on the street corners) to make enough money to fund his necessities and club fees. He lasted about 4 days and DH told him to go back to BM.

Good luck.

What did DH do about his abhorent behavior during that month? Did he force him to make the bed, do chores, treat you like a human being? You say you went days without speaking - why was this allowed to continue?

You are inviting a shitstorm into your home. SS has already shown you he does not know how to behave like a member of the household. AND your DH has already shown you he will do nothing about this behavior. I highly doubt SS will be leaving in a year. They have shown you who they are - believe them. 

Do look up the Burning Platform as you will need it.  I'd use that  as the "penalty " for not following the contract you come up with.  Make it clear that ignoring the agreement will lead to the Burning Platform.  If he won't follow that then back to mommy he goes.  Do this much for yourself.  Give yourself an  out  if none of this works.  Don't allow too much hand holding either.  Granted he is stunted from BM but he has dna from DH too.  
 

I also like to recommend DH tell stories of growing up and the failures and challenges that helped him grow.  Don't hide this from SS.  He will need to know that growth comes from the hard times.  We rush way too quick to protect young adults from the tough stuff that they can't handle much of anything.  (I have 3 launched adult kids).  

He was younger than your SS when he moved in but you can finish school earlier here.  Due to an awful family situation, he could no longer live with BM.

SS was reasonably polite and DH cleaned up after him from day 1 but I still ended up fantasizing about leaving and living in a studio apartment on my own after a really short period of time.  It also led me to this website.  I felt like I was living with a parasite who leached all my money and sanity.

Eventually YSS was kind of launched to live with room mates but the experience nearly ended my relationship with DH.  And DH wasn't a complete Disney dad and he did do all of the cleaning up after YSS.  We subsidised the rent because we live in a really expensive city.

I won't advise you one way or the other on trying to help to get your SS launched but DO NOT let him move into one of your DH rental properties.  Making him get room mates will be much better for him even if you have to help him with the rent.  If you let him go to one of your properties you might find that instead of having a non launched 40 year old living in BM's basement, you have an unlaunched 40 year old living in your rental.

I would also chime in on the letting the kid live in your rental.  I went through that with my YSD... and I LOVE my YSD.. and she was generally a good kid.. held multiple jobs down plus going to school.. hard working etc...   We had a home we had bought to flip but it kind of lined up with her getting out of school and having a job and she was looking at some semi sketchy apartments locally so her dad figured we would delay selling and give her an alternative that was cheaper than the rat holes she was looking at...  It is actually the reason why I found this site.. that very situation.  I was frustrated that I had put all that hard work into the house (I did the lion's share of the renovation by myself.. no contractors.. just me)... and I was a little bit miffed he offered her the house for much less than I thought we agreed on.. plus she didn't actually seem all that grateful for the opportunity at the time.

And... we initially had a deal when her first BF was living there with her that if she had a roomate then the rent was higher than just her.  Of course, after they broke up and she started dating another guy.... she ended up letting him move in while telling us he wasn't living there.. and keeping her rent low.  I mean.. we now know the guy wasn't paying a dime to be there anyway.. but it was the principle of the thing.. she had agreed to do something and then was shady with us.  Plus, as a young kid with little knowledge of house upkeep... she let some things go on that caused some damage when she didn't tell us about them in a timely manner (water leak).  I mean the CAUSE wasn't necessarily her.. but not telling us let it get to the point where some stuff was damaged.  They also hung pictures with what I can only describe as railroad spikes..haha.. and the home had wood paneling so that looked great when it came down.. (not).

She also had the assorted parties there.. and the house just wasn't kept as nicely as it really should have been... young kid.. a lot on her plate.. etc.. 

I will say that she DID pay  her rent though.. it wasn't a huge amount.. but she paid it and it "almost" covered the cost of ownership for us.. we did subsidize a little bit.

She has long moved out and we have since sold the place.. but it took another round of renovation to fix it back up to saleable condition.  Our relationship was not permanently damaged.. but it was a bit shaky for a bit 

I honestly think it would probably be better for the option to be to find a place for him to rent.. even if he needs a roommate or two and even if dad ends up helping him make the ends meet.. but paying a third party will be another notch in his adulting belt.

Still lives with BM.  He is a perpetually unemployed actor which means he sits in his room playing video games all day.

BM doesn't mind him living there.  I guess it avoids feeling and being alone for her.  It is clearly stunting SS though but she is selfish and wouldn't care.

SO has a nit-so-strong relationship with him but does try to encourage him to move out on his own.  Unfortunately he realizes there is no fixing this unless 1) BM actually steps up and pushes him out of the nest or 2) SS realizes he's a loser and moves out on his own.

Your SS moving in with you is not indicative of wanting to grow up.  He is probably bored at BMs and this will be a big vacation for him.

And once there getting him out will be a big issue.

My experience turned into a year and a half. The worst part is feeling your life is on pause while everyone functions around the non-contributing adult. I like to be home alone sometimes and that wasn't possible. Do we go out and eat alone or does SS always have to come or be asked? Weekends we are doing our chores and he is still in bed, so when he showed up at 2:00 to make coffee  DH is like "Hey Buddy!" and I'm like "WTH?" Meals had to include food and sides he liked, and if we went away for a weekend we had to leave SS money and/or food (according to DH). UGH, now I'm all worked up remembering it. Good, good luck. I truly hope you are successful quickly!

It's a terrible idea to have him move in. He's a big boy and if he is making zero effort to handle himself like one it'll be no different at your house. Plus you say you'll kick him out but it might be harder to get him out than you think. Personally I am on team buy your own Huggies.

Try the burning platform rather than the welcoming coddling support.  Before he moves in inform him that he must either be in school full role or working full time.Or 1/2 -n-1/2.  If not, he is your full time live in beck and call boy/chore bitch for 8hrs per day when he is in your home.

It worked like a champ with my skid who grew tired of working his ass off only for room and board and joined the USAF after 4 mos as our chore bitch.

Meanwhile back at the ranch and 10.5 years later he has a great USAF career and has completed 10 years of service.  His mom and I are very proud of him.