well there could be lots of reasons. have you asked HIM about it? i know many guy don't want to have the converstaion, but if it persists a converstaion is going to be needed.
possibilities:
he usually has one nights stands and can't handle the increased intimacy.
fear of intimacy in general.
he is unsure about you and your shared relationship
he doesn't like condoms
he doesn't like sex
he is on medication that affects his sex drive
he is on illegal drugs that affect his sex drive
the list goes on. like i say my personal opinion is that you need to have a converstaion with him and stop trying to solve a problem that might not have anything to do with you.