That sounds awful. Nobody should hit anybody ever, for any reason except self-defense. "Blackout drunk" is not self-defense. Your son has no obligation to have any contact ever with someone who has ever hit him.
And good on your son for telling his father that he cannot have a relationship with him right now. "You're all I have" is emotional manipulation, and good for your son for recognizing it as such. I'm guessing he got those strong values from you - well done, Mom!
And good for your boyfriend for being around, a counter-example for your son to show that not all adult men are raging, self-pitying drunk losers. (I imagine it must be particularly tough to deal with this stuff when it's the parent of the same sex - there must be some moments of thinking "OMG is that going to be MY future too??"
(I'm also thinking this is a potential preview of my daughter's future - her father also pulls the "you're all I have" crap, and his drinking and raging is getting worse and worse. She's still too young to stand up to him the way your son did, but I think that day will come. In the meantime I do my best to keep her safe while trying to facilitate visits with her father.
Once again, your son sounds like a strong, self-aware young man. I'm so sorry you and he have to deal with this.