You bet $20 on the turn, and now he leads out for $18? If he felt his hand was good he would have check raised or led into you for more (or check raised the turn). I have no idea what he was afraid of. (A set maybe?) I think he played this hand horribly, and got lucky.

Earlier I thought you had raised preflop, but now that I see you didn't, this looks a lot like AK to me. I would have been much more worried if he had checked it to me.

In most cases, he will win more money check raising that turn. You bet $20 at the turn and he only called. He then got $18 more out of you at the river.

If he had check raised the turn (even if it was only the min), it would be hard for you to fold. You might, but 90% of people wouldnt. He gets $20 right there, plus more if you call a river bet which most people would now that the pot is this big.

He won this one, but I think he misplayed it and you will win tht hand there often enough to make that call or even raise the river.