Either a set of nines or an over pair. He can't figure you for Aces since no reraise. Since you coldcalled for deception purposes, and this guy has no idea you have aces, this is just the situation you're looking for. If he's got the set of nines, that's just crummy luck. A large reraise preflop might have driven 9-9 out of the hand, but hey, that's what you risk when you don't reraise preflop.
Anyways, I don't know what to think about the 9-9. His pot bet could mean anything. There are 2 possible open-ended straight draws, so giving free cards would be a disaster for him, especially with 4 other people in the pot, which is what he'd do with an overpair as well as a set. He has no reason to believe you have anything larger than top pair, so him pushing with an overpair could make sense. It's tough, but seems to me like you gotta pay him off if you're beat.