If celebrity tabloids have taught us anything... it is that you have to protect yourself. I work for attorneys and consulted them about a prenup after I became engaged. Neither my fiance or I have anything substantial that isn't already joint property or already legally protected so we decided it wasn't necessary. If your fiance were the one with the most to lose, I'm sure he would want to protect himself. He probably just needs some time to think about it.

Good Luck and Best Wishes