If he is indeed semibluffing and betting his draw, then the "Theory of poker" says to give it to 'im. Reraise him, and if he stays in the hand any longer on a draw, it's his mistake.

If he reraises, I think you've gotta flat-call it, and hope (if he's weak) that you have him outkicked. If he's good, watch out and be careful he didnt flop a set. (Though I won't go broke here, you might lose a bit of money raising to find out what he's got) Evaluate the turn.