Completely baffling since he didn't raise pre-flop, not even a min-raise. I don't play much 5 or 6 max, is that more normal on those tables..? In a full ring game you're just asking someone to suck out on you by not raising. The rest of his play kind of makes sense, although it would have served his slowplaying ass right if you'd hit the flush on the river with a card that DIDN'T give him a boat.

In any event, sorry you lost the hand - happens sometimes.