grunching

Given his low aggro, he could be raising with two raggedy pair, but it's more likely to see where you are at. If he is ahead with like top & bottom pair, you have some outs in the board pairing, or pairing your kicker. If he has a weak ace, and you are worried about him pairing his kicker, you both have the same chance to pair your kickers, but you are ahead, and will win more often. If you are concerned about him pairing his kicker on /this/ river, what would would lead you to belive this? You have to bet here always, because you are more often the winner. This is probably a bet & fold to his raise, because of his low aggro. You will save a big bet here in the long run. He may have been playing his two raggedy pair passively to the river, in which case you should make a note.

edit: correction