What do you think he is raising with here? His check call on the flop is very strange to me. You would think sets would like to raise this board, bigger overpairs that slowplayed pre aswell. I guess he could check call TT, 88, AJs (if its in his range). But would he ch/raise that on turn? Either way you get a sick price on a call so it would be very bad for him to bluff in this spot because you gonna have to call with like every hand you bet the turn with. Just so unlikely that you are ahead here, so if we believe that villain has no bluffs here/worse hand - just fold. But if he ever do this with a worse hand or a bluff we have to call given the odds we are getting.

why not c-bet bigger on the flop? since you are only betting 1/3 pot you are giving all of their draws too good a price to draw. on the flop I think there are only sets that beat you because I would argue that they should not have too many two pair combos in their range (except maybe the occasional 67s). and if CO were to have a set I think he would 3-bet (especially because UTG has already folded, so there is no incentive for him to slowplay and keep UTG in the pot). And since you have shown strength by c-betting on a wet board into two players he can easily put you on an overpair and 3-bet and get it in with a set.

So if you c-bet the flop sth like 6.25 (2/3 pot) then you can shove the turn 17.12 into a pot of 21.85.

I think the turn 9 does not change too much (unless he has 99 exactly) but I think the 9 does not improve his range because he should not really have T8,J9. Which is why I would probably not check back the turn. I could however see him raise the Turn with a hand like 8c9c, QcTc, Tc9c, 88, TT. Given the great pot odds I do not think you can nor should fold in this spot.