How obvious is villain's hand? PLO 50

[Silent A]

Villain has played very straight forwardly and has been running well to build his stack from a minimum buy-in of $10.

PokerStars Pot-Limit Omaha High, $0.50 BB (6 handed) internettexasholdem.com

saw flop|

MP ($4.40)
CO ($14.45)
Button ($37.75)
<font color="#C00000">SB ($75.35)</font>
<font color="#C00000">Hero ($45.25)</font>
UTG ($44.45)

Preflop: Hero is BB with K[img]/images/graemlins/heart.gif[/img], 9[img]/images/graemlins/club.gif[/img], 7[img]/images/graemlins/diamond.gif[/img], K[img]/images/graemlins/club.gif[/img].
UTG calls $0.50, MP calls $0.50, CO calls $0.50, <font color="#666666">1 fold</font>, SB completes, Hero checks.

Flop: ($2.50) 5[img]/images/graemlins/spade.gif[/img], 3[img]/images/graemlins/heart.gif[/img], 5[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(5 players)</font>
SB checks, Hero checks, UTG checks, MP checks, CO checks.

Turn: ($2.50) K[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(5 players)</font>
SB checks, <font color="#CC3333">Hero bets $1</font>, UTG folds, MP folds, CO folds, SB calls $1.

River: ($4.50) 3[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(2 players)</font>
SB checks, <font color="#CC3333">Hero bets $2</font>, <font color="#CC3333">SB raises to $10.3</font>, Hero ... </font>

... goes in the tank ...

OK, I admit that this is a fairly rare situation, and it isn't at all deep so it's probably valid to argue that I shouldn't worry and just stack off because villain could be a complete donk.

But I find his check-raise on the river after being so passive 100% suspicious and I'm seriously considering folding.