[Ace-Ex]

Don't fold the river! If you are going to fold to a raise on the river, why not just call down? It costs the same and you might be ahead (though unlikely.) He's probably a weak-loose player with something like A-10, but it could be 8-9 suited or any range of bad hands. With no read you can assume it's worth a showdown. Any good player would have raised a decent ace or set and would not have called a bad ace preflop. A quick call indicates some sort of draw usually IMHO.