@nevermatchtoesandfingers
your update about his mothers mood swings and use of drugs & alcohol is very telling. Can you genuinely not understand how this situation has evolved? Of course your DSS has a lot of resentment, anger and is struggling. Your DH bears the brunt of that because his son a) needs someone to blame; b) carries some resentment towards him, and you both need to recognise that some may be justifiable, whilst some will not be.
However, I can’t believe you are so naive that you have not realised that this young man has almost certainly had, at the very least, a high degree of instability and uncertainty throughout his childhood. Your DH might have contributed financially, but how much did he shield him from the behaviour of his mother- behaviour which was bad enough that your DH felt he had to leave? Was he an involved father, or was the majority of support financial?
You may have absolved yourselves of responsibility for the situation because you tried for custody- and perhaps you genuinely did exhaust every possible avenue to get custody of DSS- but your DSS probably feels that he was the one that paid the price for your DH getting out of an unhealthy and unhappy situation, as he was left behind in it. He possibly (and in all likelihood incorrectly) think that his mum might have been more stable had your DH stayed with his mother and been as financially successful. He would also not be feeling the pressure to support her whilst he studies, because you would be able to do it. He probably worries about his mum, and hates that her life is so difficult (despite it being her own doing), whilst seeing his dad create a good life. I suspect he has also had all sorts of narratives dripped in his ears over the years, about his dad abandoning them to build a better life without him. He is almost certainly aware that it’s not actually all your DH’s fault, but the situation hurts him and he will have all this thoughts in the background. It will also be hard to criticise his mum to you/accept any criticism of his mum from you or DH, so he won’t want to be honest about his feelings about her. I imagine he has quite a lot of conflicting emotions regarding his mum, and his dad.
I understand that you feel defensive towards your husband, but I think you should park your (understandable) that stance on this and try to understand why your DSS is reacting the way he is. If you can do that, perhaps you can all start to work a way through it. I don’t think framing it as unreasonable DSS vs reasonable DH is helpful in this instance. You making him into the “bad guy”, isn’t going to even start to repair that fractured relationship. It’ll drive him further away, which I don’t think your husband would want.
I’m not saying your DH should have stayed, but he did leave his young son in what sounds to be a very unstable home environment- one which it he could not tolerate living in. That was bound to leave its mark on his son, but it was the choice he made. Now he has to try to undo some of that damage, sadly.