It’s hard not to get stacked here.
First of all, his UTG limp with AA is pretty horrific. My only guess is he was planning to limp-raise pre-flop. UTG limp-raises are almost always QQ+, usually KK+.
You absolutely should be raising 99 over limpers here. There are 2.5 BB out there to steal and you have a strong hand. Gotta raise this. That raise should mean he 3-bets you, which you should probably call unless he goes huge. Then, if he bets this flop I probably fold given this line. That’s about the only way you don’t get stacked here. But once you see that 9 on the turn you gotta go broke. He should have more A3 and naked 3’s in his range than AA.