I think your small value bet on the turn and river after checking a flushed flop would worry any sane player. But, his all-in bet on the river shows a complete lack of concern for the possibility that you yourself may've flopped a flush. Unless your opponent is really bad at making decisions, I don't think he can sensibly move all-in here with anything other than the flush, so I think there's a solid chance that he may've flopped the flush, possibly even the nut flush with suited overcards.