[imsomoney]

This is tricky...I would be tempted to take the free card on the turn, but since it's heads up, I don't think the bet was a mistake since you could actually be ahead in the hand and there is a good chance that he folds to that bet. From the river action, I suspect he was slowplaying a set all along (or put you on high cards and figured he would try a bluff). If you say he thought you were a LAG, the first option is more probable.