Worse yet, he hasn't heard from his friends all holidays. He assumes they've finally dumped him and his child-like body - except then he finds out that it's much worse than that. His friends are now so far ahead of him that they've started hooking up and getting girlfriends.
Jack is screwed.
But then he comes up with a plan to catch up and win his friends back. And his plan is perfect: he just has to fake puberty.