That's a scary board and gets scarier on every street. I suppose it is possible that the flop cap means he hit his set, but I'm also thinking he could have a big flush draw like AdJx.

I think you have to call the river for a couple of reasons; Villian could have been pumping a draw he missed, Villian could have something like 99 and be putting you on AK or AQ, you don't want people to think they can out aggression you HU to get you off a pot.

Showing aggression on the turn may have slowed this guy down or got him off a draw, I'm not sure and have had some bad runs being too aggressive with the 2nd or 3rd best hand so I'll let someone else speak to that.