Why isn't she watching them while he's working?

If he won't respect your feelings and work with you on a solution- than he doesn't actually respect you as a partner.

I agree on some level that spouses in blended families help eachother- but we shouldn't burden our partner either. These kids are causing you nothing but stress. It is either time for them to go live with BM and you go to EOWEnd or maybe even 50/50 week on week off would be enough of a repreave from them. 

Either way, if your husband isn't stepping up-coming up with a solution that works for you... It might be time to seperate again. Not saying divorce but maybe not living together is the best option at this time.

Take your son out of there. He can't force you to come over and watch them so just don't show up...obviously warn him. Tell him to find a babysitter. I was a babysitter/nanny since I was a teen and I'm now 47 and I just left my most recent nanny job because the kids are 15 and 13. I got paid $20/hr. Let him pay someone. Trust me, they will leave and he will be stuck again. He has to know that his kids behavior is not ok. This is not your responsibility and 20 other people are going to chime in and agree. Your responsibility is to your son. It's a beautiful day, take him to a park, a museum, the zoo and see what your life should be like. Do it before it's too late and he learns behavior from them. He will start to act like them to get his way. He sounds like a sweet boy but he will change in that environment

I thought this was priceless: 

"I've tried and tried to talk to SO about this and he remarks I'm being selfish and he's tired of talking about nothing but problems".

Pardon me, YOU'RE being selfish????  That's rich, coming from someone who expected to be able to retire to his bed each evening at 7pm and for you to get the kids to bed and off to school in the morning.  It's a shame you married this guy before you knew how the land lay.   He wants you to come back so that you can carry on where you left off, being nanny, housekeeper, chief cook and bottle washer. 

"He claims if I lived here they would respect my position as an authority figure "  yeah, right, haha, was that a flying pig I just saw?  

I think if you read your post and it was coming from a stranger you would be "oh dang.. this is a no brainer.. what the heck is she still doing with this guy?"

1.  He constantly pushes virtually all of his responsibilities for the home and kids on you

2.  Your child was suffering in the home environment.  Now, I won't say that I think that sometimes kids do need to learn to deal with things like new schools etc.. and I wouldn't give a kid that much power.. but it seemed to go beyond that.

3.  His kids are not parented by anyone but you and respond accordingly.

4.  His wife didn't have full custody is always a warning sign since the courts almost always prefer the mother over the father.. so what went on there?

You seem like you are doing well and fully supporting yourself financially.. I'm curious.. when does HE take turns watching your son so you can do things? I'm guessing never.  Look... yes married couples do help each other out.. but it's mutual and normally married couples can live together not apart due to extreme dysfunction in one of the homes.  He seems to have an awful lot of excuses for why he can't hold his end of the rope up.. you seem to be carrying both ends..and getting none of the benefit.  INstead of being thankful for your help.. he is angry you won't do more.. and is keeping his kids from a mother that wants them at the same time..

I would put this circus tent in my rear view.

Marrying someone does not automatically make you his servant. Apparently your DuH didn't get that memo. Please stop looking after his kids EVER. They have two parents who could and should be looking after them or at least finding a sitter. These kids did not become your responsibility when you married their father. What did he do before you were around? Let him go back to that. Next time he expects you to keep his kids decline politely and remind him that they have a mother...

Please enjoy your trip with your son and ... make a habit of having something to do and somewhere to go every weekend from here on.

I mean this with nothing but complete respect!!!  RUN ~ U FOOL.  I have lived this type of misery for years.  I keep going back out of fear to be alone and fail.  I was married @ 16 w/ a kid,  I never got to live a life of my own or figure out who I even am.  Still don't know!!  I am now damn near 43 years old and stuck in a situation that is horrifying, it effects so many aspects of my life.  I also have deep seeded hate toward my SD whom is the culprate of the issues that then were facilitated by her father.  There were NEVER any boundries set for this girl and it shows now @ nearly 30 years old.. no job, brand new baby, and she is about to be homeless because her BM is relocating.  I primise this will NOT get better if anything it will destroy you from the inside out.  Your son deserves to have his mom @ her best!  U deserve to be at your best, the years w/ you son will fly by and you will completely regret not being able to give him you at the top of your mom game.  Close this chapter of your book and start fresh.  FOR YOU!  I wish you the absolute best of luck.  It will be one of the hardest things to do but after the fact you will be able to breath. 

Wow. Yes, get out, and let's hope BM gets custody. He sounds like a very toxic person, straight down to trying to cut their mother out of their life. She should be the one picking up the slack here, not you. And let me guess - he doesn't want her to have custody so he doesn't have to pay her child support. 

There's nothing in this for you, except misery.

If your DH didn't make any changes after you moved out, after only three months, then he never will. He's blaming his dysfunction on his medication. And while that can have an effect and I have heard people describe it as feeling numb, they don't abandon their kids and have emotional affairs. They solve the problem with the medication so they can function. Hell, they're ON antidepressants so they can function. WAY too many excuses from this one.

But, no, none of this is your DH's fault. He probably thinks it's your fault for not "helping him" more. 

Make a plan, get yourself free of this, and keep your son on the right path.

you are validated in thinking you should divorce him. Nothing you have written about your DH sounds like he has an redeeming qualities to even entertain this delusion further.

I have read nothing in your posts that would justify you staying with this man.  He is using you, just like he used his last partner, like he uses his mother and like he'll use his next partner after you come to your senses and get out of this relationship.  At least his mother is related to his heathens - you are not. He sounds incredibly selfish, and to not send the kids to a BM who wants them makes no sense if he can't handle them without pawning them of on others. 

At least you got your son out of his house.  Now take the next step and get both of you away from this user and failed parent before he destroys you. 

he remarks I'm being selfish and he's tired of talking about nothing but problems.  I'm tired of living this way. 

Follow your gut. HE is the one being selfish.

Was this explicitly discussed pre marriage or assumed? I'm guessing assumed which was his mistake.

he is cuckoo to think anyone would want to look after his ferals.

i certainly would never stay married to someone who made home life so miserable and unpleasant... I certainly wouldn't stay married to him and live in another home like i'm some dirty mistress and mistake..

what has this man done to address the issues?? Absolutely nothing right??

Im sorry you had to go through all that. it really sucks when you realise that you wasted valuable time and resources on a void such as your soon-to-be ex.

But get thee to a lawyer pronto. I would reccomend this ASAP, although you are dealing with the stress of your fathers estate. He might get a clue what you are up to, and start doing things to sabotage. Definitely watch your credit, in case he opens accounts and takes out loans, etc. Best thing is to see a lawyer...

AND take care of YOU. Get some you time...now that you arent watching his bratty spawn, you can focus on whats important.