nl100 - KK on two ace board gets river donked by TAG

[tubasteve]

Full Tilt Poker, $0.50/$1 NL Hold'em Cash Game, 6 Players
LeggoPoker Hand History Converter

UTG: $42.60
MP: $74.20
CO: $99.50
Hero (BTN): $99.50
SB: $97.65
BB: $193.35

Pre-Flop: K[img]/images/graemlins/heart.gif[/img] K dealt to Hero (BTN)
3 folds, <font color="red">Hero raises to $3.50</font>, SB folds, BB calls $2.50

Flop: ($7.50) 5 4 A (2 Players)
BB checks, <font color="red">Hero bets $5</font>, BB calls $5

Turn: ($17.50) A[img]/images/graemlins/spade.gif[/img] (2 Players)
BB checks, <font color="red">Hero bets $12.50</font>, BB calls $12.50

River: ($42.50) J[img]/images/graemlins/spade.gif[/img] (2 Players)
<font color="red">BB bets $31</font>, Hero...




Villain is GenTaoChicken. I've got about 400 hands with him running at 21/17/3.5, seems like a pretty average TAG but I don't know if hes good or not. Is he bluffing here enough of the time to call?