68 of spades looks like the obvious possibility, and i also really don't think its out of the question for a good player to have a set here. He might have initially decided to wait til the turn to raise you, but then your turn bet makes it look like you have an AQ-AK that isnt very excited about playing for a lot of money, so he just calls again intending to push any nonspade river, since thats the best chance of you putting all of your chips in with just a pair. He will also probably have air a good chunk of the time, but I don't know the player so I cant tell you whether it was a good or bad call.