River decision against a seeminly passive player

[McNeese72]

A couple of Tags and a Lag were on the other side of the table but they all folded. I don't have any hands on the players in the hand but they all seemed very passive especially the main villain.

PokerStars 0.25/0.50 Hold'em (10 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: 2+2 Forums)

Preflop: Hero is CO with Q[img]/images/graemlins/club.gif[/img], J[img]/images/graemlins/diamond.gif[/img].
<font color="#666666">5 folds</font>, MP3 calls, Hero calls, Button calls, <font color="#666666">1 fold</font>, BB checks.

Flop: (4.40 SB) 3[img]/images/graemlins/diamond.gif[/img], A[img]/images/graemlins/diamond.gif[/img], 2[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(4 players)</font>
BB checks, MP3 checks, Hero checks, Button checks.

Turn: (2.20 BB) Q[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(4 players)</font>
BB checks, MP3 checks, <font color="#CC3333">Hero bets</font>, Button calls, BB folds, MP3 folds.

River: (4.20 BB) 9[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(2 players)</font>
Hero ??

Okay, the Q on the turn didn't bother the villain. Villain is relatively unknown but my gut tells me, that as passive as he has seemed so far, he might be one of those players who will play Ax with a weak kicker and just call down with it.

What line do you guys take in this situation? My first inclination is to bet and hope he hit a Q with a smaller kicker, but my gut is telling me he has a small Ax.

Doc