I think I can see what's happened here.
This isn't what you signed-up for when you married your husband, you (understandably) thought that you'd only live with your own / the younger children. I doubt you ever even spoke about what would happen if the older stepson wanted to come and live with you.
To compound this the stepson hasn't been easy to live with, he's selfish, probably takes the home for granted and sees living with you as a convenient option (when in fact it's very inconvenient).
In my experience all of this is actually very normal for a lot of teenagers (of course not all of them, many are lovely, collaborative, friendly and helpful) but what is really hurting you and your relationship with your stepson is that you feel like it's been forced on you, and you feel powerless to change it.
From you DSS' perspective (and I was a slightly troubled teen, so I am looking at it through my own eyes) - he probably feels pushed-out, he doesn't care about your relationship with his father (why would he?), he doesn't feel he can live with his mother (even if they get on better now), in general he probably feels quite hard done by, partly because he can feel your resentment towards him. From his perspective, what has he got to lose, you don't like him anyway?
I'm not sure what I'd suggest, because the actual answer is to develop a mutually affectionate relationship with your stepson, and it may have gone too far for that.
Have you tried treating him as if he were your own? Working together as a family to set-out a plan of action, what is/isn't acceptable behaviour (from all sides), what you do as a family and for the family, it could be things like:
- If we have our bedroom door shut, no one enters without knocking
- We all leave shared rooms/workspaces tidier than we found them (exception own rooms)
- We treat each other with respect, we're not afraid to discuss difficult things
- 'Mum' cooks for the family, if you want something else it must not negatively impact the rest of the family
- All adults pay what we can afford into the family
- Once a week we do one thing as a family, just for fun...
- Etc.
I know it sounds a bit cheesy, but it's a bit like positive visualisation - if you act like a family, maybe you'll start to feel like one.
NB: I write this in good faith on the presumption that stepson is not abusive or unpleasant