If he's such a fish, he'd probably play two pair or a set identically, and would definitely call a reraise on the river with one of these hands.

You're only worried about one hand, when he could be holding a fairly wide range of hands that he probably thinks are unbeatable. I'd say this is an easy reraise. If he comes back over the top, then perhaps just call, but don't just flat call one raise with the second nuts in this situation.