I think my situation might relate a bit. My parents split up when me and my sister were very young. I'm now the brother to a 21 year hold alcoholic sister (I'm 23) and we live with my mother. We see our dad, but he doesn't know about her problems as we don't trust him much. My mum regularly leans on me and I've adopted the sort of 'father figure' or something similar in the house since I was a 16/17.
First you can't blame yourself at all. These things just have a way of happening. Supporting your son through it all like you have is amazing, and he will appreciate that. I know I appreciate it when my nan or uncle empathise with me and help me out. Keep encouraging him to get some form of counselling. If he has that, plus your support, overtime he should hopefully understand it is not his responsibility and can gain freedom.
Unfortunately it's easy for a troubled parent to lean on their child. I'm guessing your XAH relates to your son very well and your son is good at the emotional support he gives. As long as you support him, and keep ensuring him it's not his fault nor his responsibility and making sure he has professional support there's not much more you can do. If he's talking to you, and being honest, at least you know he's okay.
I hope that your son soon realises it's not his job to take care of his father. It's a large burden. If you help him see that he has to live his own life then you've done the best thing you can do in my opinion.
Keep strong, you sound like an amazing, loving mother and he is lucky to have you for support!