I see you put "give up" in inverted commas and am guessing that that is how your H would describe the process of asking SS to shape up or ship out ... whereas you probably see it as "tough love" like I do.
Yes .... asking a "child" albeit an adult one to leave the family home is a risk. It would be naiive to suggest otherwise as undoubtedly it's going to be a big upheaval for a young person used to having things pretty easy both financially and practically - but I really do believe that if it's done fairly, i.e. after plenty of conversation, and plenty of warning (that this is what will happen if things don't improve) then it's the only option left to you (I mean a generic "you") if you don't want to spend an indefinite period of time - which could be years - being treated like a doormat and disrespected in your own home.
I certainly never "gave up" on my son - as I described before I spent a long time trying to appeal to his better nature - to little avail - and then, finally, I warned him what would happen if he didn't start behaving with more consideration. And once he left, I made every effort to show him that I still cared and was happy to help (but not do everything) him find his feet and become independent, and I made lots of effort to meet up with him and show an interest in his life. I know this is easy for me to say all this now with hindsight of course because it's something I've been through and come out at the other end with a good relationship ... but it kind of stood to reason to me that as I'd brought him up pretty well (IMO anyway) and had never spoilt him, that underneath all the bluster was a good lad at heart. Maybe it was hormonal, I don't know ... but he'd never really been a problem at all until he hit 18/19.
When I asked my son to leave it was done with love. My son probably didn't feel like that at the time but I hope he's starting to realise that I acted in both our best interests. It gave me the breathing space I needed and him the opportunity to grow up and become more considerate. I honestly feel he's a better young man because of it though undoubtedly it was hard for both of us.
Your situation is a bit different of course because you not only have a problem "teen" (I know strictly speaking he isn't but he's all but one) but the added issue of his father seemingly happy to tolerate his behaviour. Which might be all very well if he was a sod to his dad as well but when it seems SS has very clearly singled you out for his contempt, he (H) is completely out of order to sit back on his arse and not tackle him properly. I didn't have that "double" insult to deal with thankfully .... though I am all too aware of how it feels to have a partner who'll do anything to avoid disciplining non-resident kids when they need it because he's petrified of losing contact.
I agree you should pursue counselling .... I'd be very interested to hear how he justifies himself effectively condoning SS's attitude. It's simply not good enough to say airy-fairy like that he'll move out sooner or later ... I'd bloody well hope so ... but how long will that be, and in any case, that's NOT the point. Whether you have to tolerate this for one more month or two more years, him ignoring you contemptuously is completely unacceptable and if this isn't nipped in the bud then, as you say, it could fester ad infinitum and cast a horrid shadow over future family occasions.
Does SS harbour any illusions still that you were somehow responsible for his parents' marriage breaking down ? If so, quite honestly, at his age, I'd be tempted to set him straight about his mother's role in proceedings. And if he does know what happened, then actually his rudeness is even more unforgivable and H's refusal to deal with it properly seems even worse.
What does your H say when you ask "do you think it's acceptable for SS to treat me so contemptuously ?" .... I mean, what could he say because there's only one right answer isn't there.