Hey guys here I am with another hand, would like some input here

I've played this hand normally and at river I've tryed take out a split, then I get this re-raise from kinda spewy player but also understands a bit of the game. I'm not expecting any 9x here, only thing I can think is either a 6x or a bluff, if so, do we get any better here with Ace high or we only calling with the 6 in our hands here? I just don't see the button with a 9 and checks 2 streets. Well I layed this down because it's just a spot I don't understand what he could be bluffing, would like to hear ya. And yes maybe I got myself in a sticky place...

Blinds: $0.25/$0.50 (7 Players)
BN: $80
SB: $65.00 (Hero)
BB: $60
UTG: $40
UTG+2: 50
MP: $37.94
CO: $50
Preflop
($0.75) Hero is SB with AcTh
4 limps, Hero raises to $3,50, 3 calls
Flop
($14.50) 9h 9c 3s
Hero checks, 3 checks,
Turn
($14.50) 9h 9c 3s 6h
Hero checks, 3 checks.
River
($14.50) 9h 9c 3s 6h 6c
Hero bets 15$, 2 folds, BTN raises to 40$
Hero ?