River Raise ??

Live 4/8, I am in the small blind with A [img]/images/graemlins/heart.gif[/img]K [img]/images/graemlins/club.gif[/img], the table folds around to the button who limps in, I raise, the big blind folds and the button calls. Heads up to the flop. Let me give details for this player before I go further. I had played with this player several times before and he was pretty loose and very passive. He had limped on the button in similar instances 3 or 4 times earlier in this session and showed down hands that probably should have been raised ie. KJs, A9, etc. That said, flop comes:
A [img]/images/graemlins/club.gif[/img]A [img]/images/graemlins/diamond.gif[/img]3 [img]/images/graemlins/heart.gif[/img]
I bet, he calls, turn:
8 [img]/images/graemlins/spade.gif[/img]
Again I bet, he just calls, river:
9 [img]/images/graemlins/spade.gif[/img]
I bet, he raises??
My read on this guy suggested that he could have a wide range of hands, ranging from middle pairs to any A. At this point I really don't like folding at all but I start to ponder a three-bet thinking he may have AT, AJ, or AQ. What is my best play against this river raise?