Everyone has their own theories. He are a few of mine (if I did all of them you'd be lectura this for days:P)
Theory 1: Jason has been to one of the different camps before. Coach Hedge dicho something like, "I've never seen tu before in my life." Think about it, the mist, maybe? He may not know it, but he's tricking everyone into thinking he's always been there. This might be one of my stronger theories, because Hedge did mention that he was probably good with the mist because he's never seen him and Piper is his girlfriend and Leo is his bestfriend, though he has no memory of them.