i'd probably call the 300 more and see a turn at the very least... it wouldn't be out of the realm of possibility for him to have flopped a set of 3's or 4's, or even J's but with no reraise preflop i think the jacks are a little less likely. Plus like dimebags said, he could have flopped top pair with an ace kicker and thinks he's ahead and that you missed and c-bet trying not to show weakness. I'm curious, what actually happend?