I think your right about him playing his cards rather than trying to put you on a hand. It's hard to say exactly what i'd do there having not seem him play at all and not knowing already how he plays, that's why i said at the very least I'd call the 300. If I thought he was a solid player and had actually hit a set even though i'm not getting pot odds there I might take a chance (assuming you weren't small stacked since you didn't push pre or post flop) and see if the turn is a king and if not I've already put him on a hand that has me beat so I can fold there if he were to bet the turn as well. If I assumed he was your average limited experience free roll player then I probably would have put him on high pair with an A,K,or Q kicker (yeah I know thats not very specific but your average inexperienced player would have easily played any 3 of those hands there). After making that read, there is a good chance I'd re-pop it but how much would depend on my gut feeling and how lucky/unlucky I felt. If you go all in on your average noob free roll player your more than likely gonna get called and then he could hit another jack or like you said hit an ace on the turn and make 2 pair and that would cripple you. Also, if you come over the top of his raise he might just push all-in and your in the same situation as if you had pushed. This is why I said I'd at least make the call because then if your a solid player you leave yourself the ability to fold if the turn comes in what you believe to be his favor.