V was new to table, younger chatty kid, upbeat. Seemed loose but not clueless.

2 limps to him, he raises in MP, BTN calls, SB folds I am in BB with QT

I think I need to 3 bet here. I just called, all calls.5 way action, 5 BB

Flop QT4r. I check to aggressor, checks to V, he bets, BTN calls, I raise, 2 folds, V calls, BTN folds. 7.5 BB

I am trying to put him on a hand-I think AA KK is 3 betting, AQ? AK? JJ? KJs?

turn is a J. He gives me a look and a very deliberate check. maybe a FD here but I don't recall.

I hesitate and check behind and immediately berate myself.

River is a brick. No flush possible. He checks again, staring at me.

I check again. He went from chatty to much more serious and it was a weird vibe.

I needed to bet the turn but since I didn't do I now bet the river?

The action doesn't make sense. You're in the BB so you are not in a position to check behind on either the turn or the river.

As played I am fine not 3betting pre flop (though I really don't think it matters with this specific hand) and like the flop XR. On the turn, assuming it is HU between you and the pre flop raiser with you out of position, I would bet and call down from a raise. You can probably fold an A/K/J/9 river. Once the turn has gone check/check, I would lead the river and call a raise.

We should be mixing between 3bet and flatting pre, so I don’t mind the flat.

After check/raising the flop I would keep betting turn. Sure he can have AK, he can also have AQ, KK, AA, AJ, AT, KJ, KT, even 99 and T9 that got sticky and turned some equity. He probably doesn’t have sets as we block them and he should happily 3bet them as well.

This is classic “act strong = weak” live tells so I would actually be more inclined to bet after getting the stare down. If he was joking around pretending he didn’t really care what you do, I would be much more suspicious.

Holy smokes you are correct about action I will correct the hand.

Corrected action-

I checked the turn, he checked behind.

Same on the river.

I mis remembered because of his play. After showdown I didn't understand why he didn't bet river.

Bet the turn