Does he treat your mom well? Does he make her happy? Then you mind your manners and be polite and cordial.
You should think about why you don't like him. Is it because he isn't you dad? If so you should put a real effort forward and try to be friends. If it is because something about him has your "spidey sense" going off. Steer clear as much as you can be polite, and keep an eye out for why you feel that way.