grunch:

Hard to imagine he's bluffing against a three flush board and a player showing strength all the way. If he had K's Up on the turn, calling your bet instead of raising only makes sense if he's put you on a flush. That said, he may think raising with the paired board might scare you out of a small pot. I'd call. If he's bluffing he'll fold to your reraise, gaining you nothing, but if he has the boat you loose an extra big bet.