arguing with him might result in him getting angry at you, so thats probably not a good idea.
unfortunately dogs that get abused like that often get put down because they bite out of fear, you should try calling a charity like RSPCA or ASPCA(it will be called something different if your in a different country) they would find him a new home or charge your step dad with animal abuse, which might make him stop.
you could also try talking to another adult about this, and see if they can help.
edit: he shouldn't hit you, even if you deserve it, even if you hit him back, it still counts as child abuse. (if your a child, which i assume your are if your living with your parents)
another edit: I know you'll be sad to lose your dog, but you can't tell if he'll start defending himself. and besides, he doesn't deserve this treatment, try thinking about whats best for your dog and not how much you'll miss him. He needs a new home, he'll be happier when he's away from your dad.