NL50: flopped two pair w/ AK vs. nitty villain

[raiden]

I kind of rediscovered poker recently and thought I'd begin to work on my NL game a bit (used to be a 6max LHE player).
So here we go:

Villains stats are 15/5/2.5 after about 300 hands. Very nitty ans straightforward player. Plays tight also in the blinds.


No Limit Holdem Ring game
Blinds: $0.25/$0.50
5 players
Converter

Stack sizes:
UTG: $57.81
CO: $24.57
Hero: $50.48
SB: $48.12
BB: $69.86

Pre-flop: (5 players) Hero is Button with A[img]/images/graemlins/heart.gif[/img] K[img]/images/graemlins/club.gif[/img]
2 folds, <font color="#cc0000">Hero raises to $2</font>, SB calls, BB folds.

Flop: A[img]/images/graemlins/spade.gif[/img] 3[img]/images/graemlins/club.gif[/img] K[img]/images/graemlins/spade.gif[/img] ($4.5, 2 players)
SB checks, <font color="#cc0000">Hero bets $3</font>, SB calls.

Turn: T[img]/images/graemlins/heart.gif[/img] ($10.5, 2 players)
SB checks, <font color="#cc0000">Hero bets $6.5</font>, <font color="#cc0000">SB raises to $17</font>, Hero calls.

River: 2[img]/images/graemlins/diamond.gif[/img] ($44.5, 2 players)
<font color="#cc0000">SB is all-in $26.37</font>, Hero ?.


I just can't put him on a hand that we beat. I don't think he'd call with AT even from the blinds and he probably wouldn't play so aggresively with weaker ace, but I've been wrong before. [img]/images/graemlins/confused.gif[/img]
One possibility is that he read my smallish bets as weakness
and is pushing with some weaker hand but I really doubt it. It also could easily be a split.
Against a looser player I probably would've pushed the turn.

Is folding the river really bad?