It's a really really weird donk bet. I would depend lean towards calling a lot of times here (maybe 7/10). But it would depend on your image and how good he is.
Unless you have played super passively, there's a reasonable chance you would continuation bet on a flop like that... If he truly hit big (like KQ or 55) on that flop, a check to you gets him a free bet. There's no real scary draw with KQ5 flop so there's no reason to lead out and protect a well disguised monster. KNOWING that, I would not have folded so quickly.
So if I was in your situation I would think that "If he had a monster and I haven't played passively tonight, why didn't he let me continuation bet?" It doesn't seem to make any sense. However, SEEING that the flop "could've" hit my range MASSIVELY (I could have KK or QQ), I would reraise the flop, this serves two points:
1) Identify his hand. A bluff folds, random K or Q folds... So if he calls you, he got something.
2) Avoid turn decision. Your hand is marginal at this point, but his flop bet is so illogically weak... It's a tough decision on turn if you flat called him. Reraising would identify his hand enough that you can safely fold to a turn bet.
3) Free card. If he somehow called you with a marginal hand like KJ, there's no way in hell he fires again on turn so you got a free card until river (hell, he would probably even check river).
4) Allows him to make a mistake... Even if he had a reasonable monster like KQ or 55, he still has a tough decision. You were the preflop raiser, level 3 thinking says "He probably thinks there's a reasonable chances I have KK, QQ, KQ." So your raise is strong enough that even a random KQ or AK would fold.