I hope things work out for you. The counselling certainly can't make things any worse can it ? .... and though I'm inclined to say he shouldn't have to be persuaded to see a counsellor in order to insist on respect for his wife, obviously it's better to make one last attempt at sorting this before you leave - or ask him to.
Yes - I agree he should "put you first". Though it's not really about putting you first as such .... it's simply about ensuring as much as he can that people over whom he has any influence - like SS - show you basic courtesy in your own home. I doubt he'd tolerate a friend of his, for example, coming over and ignoring you, grunting at you at best. This shouldn't be any different ... yes, SS is his child, but as an adult, albeit a young one, it's about time he started acting like one - and that means having manners and showing appreciation for the fact he has a roof over his head by pulling his weight. His father certainly shouldn't be bending over backwards for him at that age, even if he were the most pleasant young man .... the fact that he does so in spite of his insolence towards you must feel almost as if the brat is being rewarded for his rudeness.
He doesn't have to be your best friend. God knows, I appreciate only too well that a close relationship between step-relatives isn't always possible .... but hello, goodbye, please, thankyou, asking you (and not just his father) if it's okay to do xyz, including you in the conversation with his dad if you're in the room and responsing in a polite manner if you address him should be the minimum expected of him. Plus, he should be helping out around the house on a regular and reliable basis and not falling back on his dad's goodwill. Your DH should enforce this ..... if he's working he'd hardly be in the gutter if he was asked to leave (e.g. after a "trial" period where he had to show he'd pulled his socks up) and friends or not, it shouldn't be too hard to find a room in a house share - where he might make friends - or to rent a room as someone's lodger ... quite a few people are looking for lodgers right now to make ends meet.
The attitude of men (it's usually men) like your DH make me spit. This whole thing of being petrified that contact will cease if their precious children are "upset" in any way .... when in this sort of context, the so-called upset is nothing more than the normal behaviour most parents expect from their children. I can't figure out why they'd rather retain the company of a spoilt, rude, unpleasant adult brat than give them the short sharp shock they actually need which might well result in short term loss of contact, but would probably mean that eventually the relationship would be better once mutual respect had been established.
In case that sounds harsh let me tell you that I asked my own (young) adult son to leave following a period where he'd continually let me down over various issues and where, whilst he was certainly talking to me, he treated the house like the proverbial hotel .... his argument being that because he paid "keep" which was a lot less than living costs for a single person out in the real world he was entitled to behave as he pleased - regardless of the effect upon the rest of us. It wasn't a knee jerk reaction, it was something I'd thought about for a long time and only after countless "talks" had got nowhere. When I finally did it I cried buckets and felt like the worst sort of mother - but he needed that lesson, and while things were awkward for a while, I made every effort to keep in touch with him, to show an interest in what he was doing, to offer practical advice about cooking and laundry and budgeting etc and suggest frequent meet-ups. I can honestly say that now he is a great young man doing a worthwhile but potentially dangerous job and I am very proud of him. He's extremely kind towards his younger sister and often shows me a lot of generosity (eg. with gifts) and thoughtfulness (eg. by asking if I'd be interested in special offers he's seen when he's out shopping). We have a very good relationship now, and indeed, around 18 months after he left, he asked if he could move back temporarily when there was going to be a few months gap between him leaving his house share and moving into a rented flat.
If appealing to the "better nature" of someone doesn't work then I really do think that sometimes - most of the time ? - they need to have the "solution" effectively forced on them. I guess with my son it was a combination of rebellion - him feeling all grown up, earning his own money and not wanting to be "pushed around" - together with the sure and certain knowledge that a few arguments where I'd protest at his attitude were a price worth paying for a pretty cushty life at home. Sounds like similar applies to your SS - he doesn't give a damn and doesn't see why he should make any effort when he's pretty sure there's never going to be any repercussions for the way he behaves.
Your DH needs to man up and sort this out. One last chance for him to tow the line and behave like an adult, or he goes. It's bad enough the son is the way he is towards you, but your husband is also being incredibly disloyal and disrespectful himself while he allows it to continue.