It could be a standard spot, but I am a bit unsure anyway.

CO seems to be a good regular, something like 24/17. I noticed only a few small leaks, it seems that he is not ready to play back light and he CB way too often and fires too many second barrels. I 3bet him preflop a few times already, as he didn't give me an opportunity to play more marginal hands with a CS and I wanted to slow him down, it seems that he is openning less often now and he doesn't try to steal my blinds as often as he did in the beginning. When I 3 bet him, he folded 5 or 6 times and 4 bet me once with AK (I folded my 56o, but there was a showdown against a weak player who won with 33.).
He has 550, I cover.

Preflop: CO raises to 20, I reraise to 66 with , all fold, CO calls.
Flop (138):
CO checks, I bet 99, Co calls.
Turn (338):
CO checks, I?

He has a pot-sized bet left, and I was unsure if I should push now or wait till the river. I think his range is PP, AK, here. (KQ and worse are very unlikely.) I think that he will fold all PP (not sets) to a turn push, I am not sure if he would fold AK (honestly I doubt).
So it seemed to me that check > bet on turn. Hopefully river blanks and he would make a call with PP.

Another possibility is to make a 'sucker' small bet of about 120, but I think he'll see through it.

All the ideas are welcome.

I shove it in. If hes pretty good, he knows when or when not to call with a flush draw.

I doubt hes going to call a river bet with 99 just because you check the turn. I guess he might bet the river with 99 though. Personally for me, the pot is big enough that I dont want to get stacked by a flush that I liet him hit for free just to try to make more from a pocekt pair when you may or may not get any more out of him anyway.

Bet 99$. This might be fancy play syndrome, but i like it for several reasons. Maybe bet even smaller. As Roy stated we're way ahead or drawing thin.

From weak players this almost always signals weakness they have a weak hand but they didn't want to check either. It sucks him in and/or gives him a good gentle little bluff oppurtunity snce he has a potsized bet left. If he considers you weak this really looks like QQ JJ scared of the K.