I have 2 pair HU, but he bets a buyin

[pokeriseasy]

Villain has been standard aggro preflop, and when he has raised he is following up with a Cbet a good amount, but we haven't played too many rivers on his button. This is his first overbet of any kind. He's also been pretty nitty postflop as he's been folding to nearly every one of my Cbets, and nearly all of my flop check raises and donk bets.

Full Tilt Poker - No Limit Hold'em Cash Game - $2/$4 Blinds - 2 Players - (LegoPoker Hand History Converter)

Hero (BB): $1,275.50
SB: $847.00

Preflop: Hero is dealt K[img]/images/graemlins/heart.gif[/img] 4[img]/images/graemlins/heart.gif[/img] (2 Players)
<font color="red">SB raises to $12.00</font>, Hero calls $8.00

Flop: ($24) 2[img]/images/graemlins/club.gif[/img] 6[img]/images/graemlins/heart.gif[/img] 4[img]/images/graemlins/diamond.gif[/img] (2 Players)
Hero checks, <font color="red">SB bets $20.00</font>, Hero calls $20.00

Turn: ($64) A[img]/images/graemlins/heart.gif[/img] (2 Players)
Hero checks, <font color="red">SB bets $60.00</font>, Hero calls $60.00

River: ($184) K[img]/images/graemlins/spade.gif[/img] (2 Players)
Hero checks, <font color="red">SB bets $400.00</font>