I think there are ways to bring the subject up without making it appear so confrontational that you will ruffle your boss's feathers. After all, you have five years of experience with the guy. Many people will act on the principle of "better the problems you know than the problems you don't." Whenever you hire anybody, there are good things about them and bad things about them.

On the other hand, if I were to say that to my boss, he would just look sad and say goodbye. He has no ability (nor does anybody above him, really) to alter my pay to any significant extent. It sounds like your boss may not have that ability, either.