This is actually a situation I love to have a big hand in. I'm not sure what your style is, but I am aggressive and will raise pre-flop with a wide range if I am likely to have position PF.
There is nothing better than having a poor player who is sick of folding his hand in the BB and raising him again with a big hand. If you are the aggressive pre-flop type, his range is WIDE here. Poor players already have a wide range, then add to it the frustration, AND add that fact that 3x the BB may seem like a discount to him. He already has a third of the bet in the pot, that will further widen his range.
Its a very standard play in his eyes, you raise in position and follow up with a continuation bet on the flop. You could just as easily have KQo as AA. His possible range isn't limited to over pairs and a possible set IMO. He could just as easily have a big card with a 9, or some type of connectors with a 9 in it. Hell, if you have been running over the table enough, he may be trying to
bluff. I understand he is very passive post-flop, but that doesn't mean a check-raise on a very standard play is a big hand.
I'm moving all in over top. Against a poor player, I would say you have the best hand 70-80% of the time. If he caught a set, then oh well. Sometimes big hands lose, but I think its a losing play in the long run to fold in this situation.