Limped Pre. Headsup against unknown (presumably a donk given the stakes) with position on him.

Flop comes Paint-rag-rag rainbow.
He checks, you fire (say 3/4 pot). He quick calls.

Turn is a brick.
Villain donkbets 3/4 the pot.

Is he ever bluffing here?

Completely unrelated: In a tournament hand I was playing while posting this, I accidentally re-raised all-in after the flop since the window popped up unexpectedly and in a horrible position.

I had , the flop was


Two opponents (who each called) had
and :Js:

The usual "crap..." went through my head as the peeled off on the turn. River: