My mother has just had a baby and her boyfriend, who has abused her, is the baby's father. if something happens to my mother, I don't want him to hurt my little brother. How can I protect my newborn brother from his abusive and idiotic father?
If you are really worried then maybe he should get a restraining order so he can't get close to you or your mother or her son. However, if you don't want to go that far you could talk to your mother about it and maybe she could take a break with him so they don't see each other that much.
If things get really out of hand you shouldn't hesitate to call the police and tell them about your situation.