I wish kids could decide what labels they use for stepfamily members but sometimes insecure adults try to dictate the terms they use. To a kid choosing mom or dad's term is like siding with one against the other. So mom should help her son decide what to do. He could call his stepfather "Dad" when at his mom's house and then respect his father by calling him something different when at dad's house. Whatever the boy decides take the pressure off. Tell him he matters to you not the labels.