I think he is likely to limp raise AA, KK preflop? There is a chance he could have QQ and Ak in his preflop range. I have found that some of the same players that are passive and limp preflop also do not fastplay their big hands postflop.

For that reason I think he is unlikely to have a set on the flop. However against a range of flush draws (AQcc, AJcc,78cc, 67cc), straights (78, KQ), overpair (QQ), two pair (JT, J9), top pair (AJ, KJ) and a ton of pair + draws (Q9,QJ,J8,98,T8,88). Against this range you have roughly 37% equity with AT. This is when he doesnt have KK, AA, JJ, TT,99 in his range due to preflop action (add those in and you have 32%).

The problem is that you are only ahead of flush draws and there are still two cards to come. Think of you range here. You can still have all the monster hands from above. This is very near the bottom of your range and it's not really the end of the world if you have to fold it as you have so many big hands in your range, on this flop. Just think of all the combos of KQ that you have and when you flop the nuts, he'll throw all his chips at you. You also have JJ, AKcc, AQcc etc etc…