Bottom set facing donk bet river after C/C flop and turn 150BB deep

[Mr_tanou]

CO has a donk style since I arrived at the table, playing 50/20/1 for 40ish hands, so he's quite passive postflop.
I took from him two medium pots 3-Barrelling with good hands, but he doesn't know that since he folded river both times.
During that hand, he waited for some time calling flop & turn and bet the river rather quickly.

What are you doing here ? Shove/Raise(how much)/Call ?
Observe stack sizes too. How do you generally interpret this river bet ?

Everest Poker No-Limit Hold'em, $1.00/$2.00 (HH Converter by Kreatief)

MP3 ($228.15)
CO ($359.87) <- Villain
Button ($302.65) <- Hero
SB ($314.35)
BB ($185.00)
MP2 ($186.05)

Preflop: Hero is Button with 3[img]/images/graemlins/diamond.gif[/img], 3[img]/images/graemlins/spade.gif[/img]
<font color="#666666">2 folds</font>, CO calls $2.00, <font color="#CC3333">Hero raises to $9.00</font>, <font color="#666666">2 folds</font>, CO calls $7.00,

Flop: 3[img]/images/graemlins/club.gif[/img], J[img]/images/graemlins/heart.gif[/img], 8[img]/images/graemlins/club.gif[/img] ( $21 )
CO checks, <font color="#CC3333">Hero bets $14.00</font>, CO calls $14.00,

Turn: 8[img]/images/graemlins/diamond.gif[/img] ( $49 )
CO checks, <font color="#CC3333">Hero bets $36.00</font>, CO calls $36.00,

River: K[img]/images/graemlins/diamond.gif[/img] ( $121 )
<font color="#CC3333">CO bets $50.00</font>, <font color="#CC3333">Hero ???</font>