I totally agree with the hive about not finishing his words. Further more you can help to create a speaking friendly environment. Basicly give him the feeling that he doesn't have to rush. Make short pauses after your statements, if you don't aleady do. So he has a chance to respond without any time pressure. Slow down a bit, if you are a really fast speaker. It also helps to ehold a normal eye conact. Chances are he can't hold eye contact during a stutter, but reastablishes the eye contact afterwards.