Do you stack off here?

[JSnoop]

Full Tilt $1.00 Buyin
Blinds 50-100

Hero 8500
CO 4900
BB 12400

Hero is UTG+1 with A [img]/images/graemlins/club.gif[/img]K [img]/images/graemlins/club.gif[/img]

Hero open-raises to 400, CO and BB call.

Pot is 1250

Flop: K [img]/images/graemlins/heart.gif[/img]2 [img]/images/graemlins/spade.gif[/img]2 [img]/images/graemlins/diamond.gif[/img]

BB checks, Hero bets 1125, CO shoves for 4500, BB flat calls. Hero???

I feel as though I have CO beaten - he has been very aggressive, I put him on probably a weaker K, but BB, with the biggest stack at the table has gotten there by calling with garbage out of the blinds at least 3 times and stacking opponents. What does his flat-call on this board mean? It would not suprise me to see him show up with A-2 here at all - do you just stick it in, or fold this down?