That’s a heartbreaking post. I am so sad for you and especially since you - and your Son - have suffered so much loss in recent months.
Your first instinct, and throughout, has been to try your very best to help him, support and motivate him - and keep funding him in the hope that he’ll be able to turn his life around. But to date that’s clearly not happening and it may even be that in sorting out much of the financial mess he keeps attracting, you could almost be enabling him. Allowing him to create further debts.
In no way am I judging you. In your shoes and desperate about where my Son’s life was going, I would probably try to do even more. But I think that would be wrong, to be honest. I think it’s time for a re-think and to involve others who aren’t emotionally involved and whose involvement might turn this around. Whilst your Son seems to be blocking out all that’s wrong and not wanting to co-operate in any sensible plan, and he’s an adult so you’re not responsible for him, your DH is clearly at the end of his patience, and his GP (of course) is in no position to have you involved in his patient’s affairs, perhaps you could give this idea some thought.
Your Son’s medical advisors, whilst being unable to give you information, would certainly have to take some note of your genuine concerns and - given your Son’s childhood diagnosis, his family history and the losses that he has now suffered - if you informed the Doctor (by mail,even) that you now have great concerns about him, at least you will have made a start on getting some help. You are very articulate - you will easily be able to provide the info and concerns that his GP should need to address.
It’s likely that if you suggest (and your Son is willing) that some professional counselling could make his life better and tell his Doctor this is what you feel he needs, and your Son agrees) you’ll probably get no objections raised. Spend some of the money - that you may be tempted to throw his way - on private Counselling sessions. You could pay those direct (so he has no excuse not to afford them!!) Do some local research and find a good Counsellor. You’ll wait a long time before his turn arrives through the NHS. Maybe with some professional help and an independent listening ear he will start to improve and be strong enough and motivated enough to deal with each challenge and change his life. Otherwise, it does look as if you will just keep on putting sticking plasters over something which isn’t healing...or worse, ruin your own life with worry because you just can’t fix him.
Be strong, be firm, don’t give up on him but choose your moment and if he’s in a mood to intimidate, just change the subject or leave him be. You are trying to help and it’s because you care. Hopefully he’ll be able to see that eventually. Best of luck.