I dont like free cards especially when he is calling all the way and there is a good chance that he will fold. Plus, this is a kind of the flop when every new card could help him aswell as help you.

I agree that most likely he had something valuable, at least 2 pair, more than half of his good hands beat hero's hand.

I would fold on the river too if I'd know what kind of player he is or didn't care about it.