I would have lead out on the flop. There are a number of draws on the board and he may just decide to take a free card with K-Q, Q-J or two diamonds if your plan is to check-raise. Betting also helps to disguise your hand since most players expect you to check a set, and is an excellent way to extract maximum value from hands like A-K or A-Q (which are the most likely hands anyways). If you're afraid he'll fold to a bet, consider that if he'd fold to a bet, you can't expect to get much action out of him otherwise.