Need help....dont know what to do???

Sorry to hear about that, it seems like your mum doesn't want to admit that her boyfriend is dangerous. It's a common thing, I just wanted to say that...
I think you are doing the right thing by ignoring him; he could snap at you at any time, and a heated discussion with him wouldn't do any help at all.
I think you need to talk to some professionalist in the area of abuse around the house, they could assist you further with what to do.
Sorry I couldn't be too much of a help.