If his family normalizes this behavior, he believes it’s normal. You don’t have the power to change that in him. This is a problem that needs professional-level help. Don’t feel guilty if you don’t feel equipped to handle it. The enormity of this multi-generational issue is so far beyond just you.
It’s okay to break up with him. I think you’ll be better for it, I think your future children will be safer for it, I think you’ll be much less stressed out. It’s not your job to try to convince your BF how fucked this all is and I dont think it’s worth trying to seperate him from his family in order to try to build a life with him. If he had half the conviction you have about this, he would have suggested cutting off his family a long time ago.
Pop a hello to the police if it’s worth going to them about and then get as far away from that family as possible