No idea, maybe he was waiting for his powers to develop, but I wish I hadn't bothered to go and see it,what a disappointment,nowhere near as good as the previous films.
My other half is reading the book at the moment which is the full story including all the bits that were cut out of the film and that might explain it more. I'll ask him and let you know if there's a bigger story behind it.
I wasn't overly impressed with the film either and think that not having Kate Bosworth in it would have made a big improvement. That was just wrong!
Like in comic book lore, Clark did'nt have any powers until he reached teenage years so maybe its safe to say that as following this tradition (and a nod to the comics ) that Jason would have the "traits of the father" and not gain an invulnerability until later on.
Then again it could just be the effects of having the Kryptonian and Human dna inside of him.