100NL: Nut flush draw and 1 overcard on donkpush

[Aero1441]

CO is a complete donkey that CONSTANTLY open-minraises and openlimps on button. He then folds to any kind of bet/cbet from me on the flop. VERY passive and almost never bets himself. Although he's so horrible, I never see him openshoving anything (he hardly ever bets).

SB: $107.85
BB: $39.00
UTG: $94.60
UTG+1: $104.40
CO: $95.58
Hero (BTN): $154.25

Pre-Flop: A [img]/images/graemlins/spade.gif[/img] Q [img]/images/graemlins/spade.gif[/img] dealt to Hero (BTN)
UTG folds, UTG+1 folds, CO calls, Hero raises to $4.50, SB folds, BB calls, CO calls

Flop: ($14.00) 2 [img]/images/graemlins/heart.gif[/img] K [img]/images/graemlins/spade.gif[/img] 6 [img]/images/graemlins/spade.gif[/img] (3 Players)
UTG checks, CO checks, Hero bets $9.00, BB folds, CO goes allin for $91.08, Hero ?

Is this a call? I'm 50/50 against a K here right? But behind against 2 pair/set. Do you just lay it down here? What if CO had a smaller stack?