How do you know the aren't in his favor? You have no Idea how much money is in the pot. Here's the scoop if you think that top pair will stand up you have 6 outs of 47 unseen cards or roughly 7-1 againt hitting a king or ace in one card. Therefore if it's only one bet to you and there is more then seven small bets in the pot (quite likely as you raised pre-flop). You have the odds to peel one more off.