If my boyfriend's dad was abusive, Is my boyfriend gonna be abusive too?

There is a possibility, but it's by no means certain. Talk to your boyfriend about it, and see what he says. By which I mean talk to him about what it was like seeing his mom being hit, dont accuse him of being a potential abuser. If he is aware and able to talk about his feelings, then it's unlikely that he's going to blindly imitate his father's behaviour.