Capped with Underpair on flop??

i raise preflop with A9s and get reraised so i call. (4 other callers along for the ride.) flop is AK9 with K of my suit. check to me, i bet, raiser raises, everyone folds and i reraise, he caps it. turn is Q. i bet, he raises and i think 'ok, maybe he's got AK or AQ ill just call'


river is K


i check call.


he turns over 77???


how do you know when you should be collecting that many more bets from some idiot on here? i hadn't seen him play a hand yet, so i didnt realize. jesus christ. he probably would have gone for a lot more bets if i were more aggressive...