In getting to the point where he is able to swim through the tunnel, Jerry has vastly improved his feelings about himself and he has greatly reduced his dependence on his mother.
When the story starts, he has very little self-confidence at all. You can see this in how he acts all stupid trying to make the local kids notice him. Even up to the last minute, Jerry is not really sure of himself. He is really planning not to try the swim, but all of a sudden he decides to do it.
Once he manages to swim through the tunnel, however, he is much more sure of himself. He has no need for the local boys. And he does not even have to argue with his mother.