Welcome!

I agree with Jasper - at age 20, there isn't anything you can do to change the kid's behavior. Don't forget, the woman to whom you are married raised him to be an entitled baby man...

I take it he lives at home while attending college?

Sounds very familiar. I do believe that there should be some expectations for every person living in the home regardless of whether they are step children or not. It's very difficult to expect this if he has never had the expectations in the past. You need to sit down and have a very sincere talk with your SO. It sounds like he's basically a good kid just unmotivated and has no responsibilities put on him. She's not doing him any favors by codling him. This is what I don't understand....why do people not want their kids to become responsible. Don't parents ultimately want their children to be independent? As far as him taking an interest in you and what you do, well quite honestly, kids only care about themselves in this day and age. I wouldn't take that personally.
It may be that you have to pick your battles. What I would consider is, are you willing to be with someone long term that treats her child this way? Will she ever make him step up to the plate? If not then you have to decide what you are willing to live with or not live with. Very difficult.

I had a SD move into my home (I owned it) right after soon to be exDH and I married. I swear they all act like they are 14 when living with a parent.

The mistake I believe I made, was thinking I had to go through my DH to have SD be expected to help around the house. That did not work and led to many arguments between DH and me.

My advice, is if you want SS to help around the house, start asking/telling him to do so. I do not think any adult child is capable (mental block) of looking around the parent's home, seeing what needs to be done, and just doing it. But, a reasonably good kid will help out when asked. Just keep your tone friendly and matter of fact...."SS can you put the trash out", "SS can you unload the dishwasher", "SS your bathroom needs a cleaning, go do it".

Otherwise, take Sally's advice and move out.

Does she need the money you contribute to the household? Would it put her in a bad spot if you were to move out? I hate to say it but if that is the case I bet if you tell SO that you want to move out BC you can't agree on the chores the adult child does that her as* drops in to high gear and things change in a positive way for you.

I would say it something along the lines of:
Listen I feel like the adult skid should contribute to the household by helping do _________________ and I know you don't feel the same way. I've been thinking that instead of continuing to rock the boat and feeling like what I say and feel isn't important I am going to go ahead and get my own place. By getting my own place I feel like the negative feelings I am harboring will go away and we can see each other when we have time to do so. Once your adult child moved out we can talk about me moving back in.

I bet she shi*s kittens if she depends on your financial contribution. It may seem like hitting below the belt but sometimes I truly believe when you have tried so many times to get someone's attention with no luck that hitting below the belt is the only way to go. BUT you can't pull your ace out of the hole and not be prepared to really move out.

I think a lot of this kid's behavior is not too outlandish or unusual for a 20YO millenial boy. I hear my coworkers complaining about the exact same things of lazy/messy kids.

Mom didn't raise him to do much around the house. Apparently she did raise him to get good grades and be respectful. It's on her that he doesn't do the other stuff and honestly, if it's bothering you so much, close his door. You came into his life late in his teens, he doesn't need another dad and if he didn't know right from wrong by then, you aren't going to teach him.

What I will say is that at his age, getting good grades and not being a PITA around the house is not too bad. Now, if he were 20, not in college and not working and playing video games and having his bong smoking friends over.. well, you would have a bit more traction in the complaint department.

I think you need to disengage from this kid in that you shouldn't be his keeper. As long as his mess and all is not interfering with your space, ignore it. Don't resent what mom or his dad spend on him as long as you are not coming out of pocket to make up some shortfall. Some kids have it easier than others. The real world will come around soon enough. I would just make sure that the exit plan is in place for when he does graduate so he doesn't malinger around for years.

I completely understand why OP is feeling resentful of an adult kid who is getting everything handed to him. Summed up in three words: Lack Of Values.

"Mommy" is the one who instilled it in him and mommy is also guilty of it. If she wanted her son to grow up and be a man, she would have set down some household and other rules to include working and paying for things he wants. Thus, you and your SO have some differences in fundamental values.

If a person has worked for everything they've had in life they have a much different perspective than someone who has had their housing, education, vehicles, etc. provided for them. Sounds like the OP is a self-made man, and watching his partner in life not exhibit those same values towards her offspring can certainly be frustrating.

While many say move out and wait till skid launches, I don't think that will be the end of it. We are living in the Age of Entitlement and these adult kids will keep coming back to the trough because they don't know any better. This kid seems like he falls in that camp.

There are many, many people out there who have come up via the bootstrap method and are raising their kids to have the same values they were raised with. Maybe you will be happier if you find a woman who has successful adult kids, and who will have the emotional and financial freedom to truly share her life with you. Her kids will also be confident enough in themselves and happy in their lives that they will welcome a partner in their mother's life, will show interest in you and get to know you. That would be a better scenario than the dead end you are facing from this flopping fish of a kid.

Recommendation: Find someone who has the same values you do.