He could easily be on some sort of draw here if he is a loose player who opened in the CO to perhaps try to pick up the blinds, I would say he doesnt have a K, because he prob would have raised at least by the turn to test out his hand strength. I would say he has an Ax type of hand and you may still be good. A jack or queen is a very obcious possibility but if he raised with a j or q, you might be looking at him to have paired the 10 as well...
I would have check called the river although a close second is betting the river here because you may have some fold equity and Im almost positive the river card didnt help his hand, and he wasnt strong enough to raise you in the first place.
cdl