I felt compelled to write as your son's experience is quite similar to mine and I thought it might help you to have some insight.
I was a little older than your DS but I was a vulnerable individual at the time, and looking back, was groomed by somebody in a position of authority. At the time, I felt completely in love, and broken-hearted, and as if I had acted with agency. I was too young and naive to realise how I had been played, and preyed upon. It's taken me many years to realise that, so it might be better, for now, to treat your son as broken-hearted rather than traumatised, per se, because it might resonate with him more....
As others have said, you can't force him into therapy, but you can just be there for him, not judge him, not make him feel worse than he already does. Be a safe space and a support.
The other parallel is my father was also horrible to be around; it's one of a number of factors that made me vulnerable to male attention, and it made my whole life pretty miserable. I wanted nothing more than my father to be out of my life, but there was nothing I could do. Unfortunately, my mum, although kind and loving, also buried her head in the sand (perhaps influenced by my father being the dominant one) and neither of them really explored why I was drinking alone, spending so much time in my room (self-harming), and when they found out about this "relationship" punished me for it and never spoke to me about it again.
Back then, I couldn't have told you why I was in so much pain, because nobody really validated my feelings and everything was just too much (on top of the standard teenage hormones!) It's only now, 15 years later, that I can look back on events in my life and understand why I made decisions I made, why I treated myself the way I did, why I got into the relationships I did. I don't think therapy would have helped me then. I just needed to feel safe, and having a bully in the home and a lack of any real attention was not safe for me to explore what was going on psychologically.
So, my advice to you would be not to pry or push, but just to support. Develop a relationship with him that, when he's older and maybe has hit rock bottom and is ready to face his demons because he knows he needs to do it for him, not for you, that he knows he can come to you and you won't judge him or shame him or punish him. Let him know you'll accept him no matter what, and that you love him. If you can get him away from his father, I'm sure that would be hugely helpful as well.
I say all this as a female, and I can't pretend to understand the inner workings of a male teen (!) but I hope this has been useful. It sounds like an awful situation for the family, for you, but you sound like you want to do the right thing, so you should be proud of that.
