I would ask myself why villain's flop and turn bets are so small, then get slightly bigger on the river. What is the villain's aggression factor this would help a lot in your decision. And his bets are pricing you in. Only hands that beat you are JJ,44, 66, J6, 64, 56, and 55, How many of these hands would a 26/8 raise with? And would he cbet a set on that flop? Wouldn't he 3bet shove a FH on the river? As played is fine.
I like calling in position here as a 3-bet would fold out too much of his range if he tends to call light.
a weak bet from a weak player here means that he often has medium pocket pair / a weak draw here / maybe a weak Ax or Kx hand sometimes.
If this was a 2 player flop calling might be ok here sometimes but with another player yet to act you really need to raise this flop.
This is a very small bet making his range very polarized. He might be doing this with a weak flush but since you didn't raise the flop, I don't think that there is much value to raising the turn here.
His bet size noticeably increases here on the river. I'm expecting him to have caught a 2nd pair / complete his flush / have a FH / feel that his medium pocket pair is the best hand here.
I just call here, raising is very thin against a very passive player who might have been trapping us all along.
When he shoves over your raise I'm folding all day long vs. this kind of player. His range consists of a FH far too often to call and hope he has the K high flush.