Hardly passive.

Would he push this with a straight/flush? Possibly.

An overpair? Probably.

A lower set? Probably.

AJ/KJ/JQ? It's $25nl.

The turn bet is suspiciously small, and after you've shoved a sign in his face to tell him you have a very big hand, too. This in itself doesn't mean a whole lot, as many players at these limits don't really understand bet sizes. What would be a better focus is why is a tagged passive player suddenly betting every street? The label passive doesn't put him on a 6 or 9T here, although I would have seen worse.

So what are we worried about? AKc and JJ, which must have a decent chance of being true with the betting until now - most players will go with their overpair here and many will stick with their overcards at this limit. The small turn bet would have given him a cheap river card, and in which case he didn't play it so badly with the exception of the flop call - a call which could easily suggest an unmade hand.

There is also a case to suggest that he has very little and is making a desperate last ditch attempt to rep a scare card and take this pot. If you can't credit him with the ability to think away from his own hole cards, this is very possible, because why would you want to scare out a lesser hand now? Keep the bet small enough for him to call.

Given what we know about him and the action until this point. QQ and JJ seem the most likely of his holdings. How often does he need to have QQ here? A little a under 2.5-to-1 bet, and so if he's packing QQ half the time, you're coming up trumps.

In terms of what you could have done to improve your decision (or at least odds) here, a raise on the turn seems good - perhaps to 10, you don't want to push him off of it if he really wants to play, but you want to make him calling a mistake, which translates directly as long term profit. I don't think you're hugely worried he's holding a 6.