Throwing money after a bluff when should I stop or keep going?

Villain is 28.6/4.8/0.67 over 21 hands but already up and down a lot, seems fairly loose. I figure I can represent AK at this flop, but he called. Should I have stuck with the bluff, should I have not made it in the first place. Let me have it...

Paradise Poker No-Limit Hold'em, $0.50 BB (10 handed) converter

UTG ($44.50)
UTG+1 ($55.25)
UTG+2 ($21.50)
MP1 ($24.25)
MP2 ($51.00)
MP3 ($48.50)
CO ($81.50)
Button ($31.25)
Hero ($44.00)
BB ($30.75)

Preflop: Hero is SB with A[img]/images/graemlins/heart.gif[/img], J[img]/images/graemlins/spade.gif[/img]. Hero posts a blind of $0.25.
<font color="#666666">3 folds</font>, MP1 calls $0.50, MP2 calls $0.50, <font color="#666666">1 fold</font>, CO calls $0.50, Button calls $0.50, <font color="#CC3333">Hero (poster) raises to $3.5</font>, <font color="#666666">1 fold</font>, MP1 calls $3, MP2 folds, CO calls $3, Button folds.

Flop: ($12) 2[img]/images/graemlins/diamond.gif[/img], K[img]/images/graemlins/spade.gif[/img], 7[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(3 players)</font>
<font color="#CC3333">Hero bets $10</font>, MP1 calls $10, CO folds.

Turn: ($32) 5[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(2 players)</font>
Hero checks, MP1 checks.

River: ($32) 9[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(2 players)</font>
Hero checks, <font color="#CC3333">MP1 bets $10.75</font>, Hero folds.

Final Pot: $42.75