[LearnedfromTV]

His first raise in an hour means his first raise in about 35-40 hands? If his cards have been average during this time, we can speculate he's raising the top 2-3% of hands, speculation that's really rough and probably low, but given that he's UTG, maybe not. That he called a short stack with AT is pretty meaningless in thinking about his opening raise range. He hasn't opened a pot in an hour is what gives us a read. He could have been card dead but we have to use the information we have. Plus, again, he's UTG.

So it's AA-JJ, AK 90% of the time. Maybe a little TT-99 or AQ. You have to call and play poker. You are getting implied odds to flop a set vs an overpair and you have position which you can use to smell out a missed AK. There is some risk involved in allowin others into the pot, but since you are drawing more often that not, I think bloating the pot for the times you hit a set is more valuable than isolating for the times he has AK, especially since AA-KK and maybe QQ probably push you off your hand preflop (and you lose 25% of your stack) if you reopen the betting.

Flop plan depends on too many factors, but I think you'll be folding more often than you call and calling more often than you raise, if no J flops. On a favorable but dangerous flop a raise enables him to play perfectly against you; seeing the turn cheaply, if you can, gives you a lot of flexibility to read the board and his actions and use your position to either identify that you are likely to have the best hand or push him off the best hand.

Edit: I just saw adanthar's point that a three or four way pot makes UTG's postflop actions more readable. I agree.