At the beginning of the story, Jerry is on the brink of puberty, trying to separate himself from his mother and prove himself as a young man. He's also a lonely young man who has no male role model or male friends in his life. He feels like he's a "mama's boy", and he wants to become a man.
By the end of the story, Jerry has matured because he's able to swim through the tunnel by himself, showing he is ready to venture into the world as a young man. Because he was willing to face the dangers of the tunnel, he has proven to himself that he's ready to separate himself from his mother. His desperation to be with the other boys at the beginning reflects his loneliness, but by the end, he doesn't care about the boys.