[jeffraider]

He probably doesn't have a big pair.
He could easily have limped with 95s or 92s or whatever.
People check-raise with top pair all the time. Who cares if it's a good play or not, he could have done it.
He probably doesn't have a set.
He could very easily have a PP here like TT-33.

There's nothing to think about. You made a suspect (but not terrible) semi-bluff (more like outright though, you only have a gutshot) and he raised you all-in. Are you hoping he has 7-high? If he's got A-high then you could be as good as a 2:1 dog, which is your absolute best case. Any other case has you drawing very slim, so I don't see why you're so interested in this hand.

EDIT: I could make the same play he did with a set, two pair, a strong 9, a PP 66 or better, a 5 with a decent kicker, etc. Nothing you've got a ghost of a chance at liking, but I'm not the villain.