How do I follow up a job interview?

I wouldn't e-mail him, I would call. Give him a couple days to do other interviews then call. First thing you should say is "thank you for your time with the interview the other day." If he doesn't chime in with information, ask him how the interview process has been going. Either he will say he is still interviewing or he has made/is making a decision. If you two have a personal relationship he should be straight with you and tell you the truth.