I would put him on 5/6, AQ, AJ or something like that. I don't see AK because he would call on the river more than likely. As I'm writing this I'm leaning more and more toward 5/6. He bets about 1/2 pot on the flop, which I would take as a draw / bluff. I like your action on the river to a really weak bet in comparison to the pot. Good read = a good play on the river.