Has he ever had therapy?
My partner used to get pushed down stairs and have black eyes going to primary school from his father when he was a little kid and given money then so he wouldn’t tell anyone. Sometimes he cries telling me about it.
A lot of people use drink as a coping mechanism, my mum was an alcoholic, but strictly in the evenings so it 'didn't count' to her, she went through a really traumatic time a few years previously, well we all did actually but she bore the brunt, and she never really got over it all, I think it was her way of numbing everything but when she was drunk that anger at the injustice of it all used to seep out of her pores and she seemed annoyed, she would be cold and sarcastic if not downright argumentative over nothing.. I just avoided her in the evenings so I didn't have to engage.
The worst part was the next morning when I'd come downstairs and she'd be my fantastic lovely mum again, she either never remembered or she pretended she didn't, and I would feel incredibly guilty for feeling annoyed with her the previous night.. every night and day that cycle happened, every night the anxiety of which version she would be, every morning the guilt, I was the same age as your son when this all started.
She never stopped and she died by 60, I sometimes think if she had therapy she may have been able to let go of some of that anger and be free of the coping mechanism, but she never did. You won't be able to 'help him' OP, he can only help himself with your support if you choose, but you should not put your kids through everything that alcoholism entails.