Help! Pet and stepson trouble!!

I have the sweetest dog you could ever hope to have but he has finally had enough of my stepsons antagonistic ways. He is forever annoying him and being unkind to him. Yesterday my dog bit him on the cheek but it did not break the skin. How can I address my step sons bad behavior? My dog loves everyone and is not aggressive. He is just tired of being scared, hurt and annoyed and I don't blame him. I protect him as much as I can. If you are going to suggest I get rid of my dog, don't waste your breath. Thanks!