CO is a total whale: 80/43/23.1 - 90 hands

I'v just been value betting the crap out of him, this is the first time he's raised one of my bets, although he was betting a very weak range when I checked to him OOP.

Call or fold?

PokerStars No-Limit Hold'em, $0.05 BB (6 handed) - PokerStars Converter Tool from http://www.flopturnriver.com

SB ($9.77)
BB ($12.45)
UTG ($5)
Hero (MP) ($12.32)
CO ($6.45)
Button ($13.59)

Preflop: Hero is MP with K, K
1 fold, Hero raises to $0.20, CO calls $0.20, 3 folds

Flop: ($0.47) 3, 8, J (2 players)
Hero bets $0.35, CO calls $0.35

Turn: ($1.17) 7 (2 players)
Hero bets $0.75, CO calls $0.75

River: ($2.67) 7 (2 players)
Hero bets $1.70, CO raises to $3.40, Hero??