Why does Jerry keep his feat from his mother in "Through the Tunnel"?

Jerry does not tell his mother about the feat he has accomplished—swimming all the way through the underwater tunnel through the rock—because he understands it would make his mother very worried about him. He recognizes the somewhat difficult position she is in (as the single mother of a growing son), and he understands her ambivalence about giving him increased independence.  Early in the story, Jerry can tell that his mother is thinking through what he may or may not want to do and whether he might want to go to the beach alone, without her.

She frowned, conscientiously worrying over what amusements he might secretly be longing for, which she had been too busy or too careless to imagine. He was very familiar with that anxious, apologetic smile. Contrition sent him running after her.

Jerry knows his mother feels anxiety over him, and he has no wish to add her to worry; in fact, he seems to feel an obligation to soothe and assure her. Telling her about the tunnel would only make her worry more.