After his rejection by the older native boys, who have perceived him as immature as he splashes foolishly in the water, Jerry sits on the rough rock and cries "openly" because he envies their camaraderie and freedom and daring. So, after having "cried himself out," Jerry swims back to the beach where his mother sits, and demands swimming goggles, nagging her until she takes him to a shop and purchases them.

Equipped with the goggles, Jerry returns to the large barrier rock where he dives down to the base of this rock, but he can find no gap in it. Nonetheless, he perseveres until he does discover the hole that the boys have entered. And, because the rejection of the older boys has made Jerry feel childish and inadequate, he feels he must be able to go through this hole himself as they have done. Therefore, he secretly practices holding his breath until he becomes confident that he, too, can swim through the tunnel and, thus, in this "rite of passage," feel the older boys' equal. In addition, Jerry must prove his manliness to himself as well, so he challenges himself in facing the danger of swimming through the tunnel. 

As he swims through the dark tunnel, "[V]ictory filled him. But, soon his lungs begin to hurt and he know that he must continue in the darkness or drown. Finally, while he lapses in and out of consciousness. Jerry makes his way into the open sea and struggles onto the rock where, exhausted he lies for a time. When his heart quiets down and his eyes clear, Jerry sits up and watches the other boys; this time, however, he does not need them because he has attained maturity and independence:

[H]e did not want them. He wanted nothing but to get home and lie down.

With determination, Jerry has proven to himself that he can do what the older boys do; he, too, is manly and not a child, anymore. He returns home and proudly tells his mother that he can hold his breath for nearly three minutes.

At the end of "Through the Tunnel," why do you think Jerry tells his mother how long he can hold his breath but avoids telling her about his swim through the tunnel?

In Doris Lessing's short story "Through the Tunnel," Jerry keeps his swimming accomplishment to himself perhaps because he doesn't want his mother to know what he has been doing (so she doesn't scold him or worry) and perhaps also because this feat has been a personal achievement that he wants to reflect on for a while.

First, Jerry definitely has a sense that if his mother knew what he has been doing, she would stop him. After Jerry spends time with the local boys and realizes that they are swimming through an underwater tunnel, he decides that he must do it, too. He cries at first because he can't and because the boys seem to reject him for it. But then, the idea takes hold of him, and he actually begins a training regime that is unusual for an eleven-year-old boy. He builds up his ability to hold his breath underwater, working at it gradually over several days. Yet he never tells his mother about it and downplays the problem of the bloody nose he gets at night. One day, his mother insists that Jerry stay with her, and this drives home the point that he should not tell her. After all, Jerry's mother worries about him. He is an only child, and his father is dead.

Jerry doesn't even tell his mother after the fact, for again, he realizes that she wouldn't like what he has done. She may not allow him any future freedom (for she has struggled enough with letting him go on his own this time around). At the very least, she would probably give him a sharp scolding for taking such a risk, and she would worry all the more about him next time. Jerry does seem to love his mother, so he would want to avoid that.

Further, Jerry wants to keep his accomplishment to himself so that he can reflect on it for a while. He has done something major here by swimming through that tunnel, yet it has scared him a bit, too. He realizes both his new-found strength and the danger in which he has placed himself, and he needs some time to process both. He must meditate on his reaction to his achievement (which is likely not quite what he has expected) and decide what it might mean for him in the long run; and this is something no one else, not even his mother, can tell him.

In "Through the Tunnel," why doesn't Jerry tell his mom about swimming through the tunnel?

Jerry's having swum through the tunnel is his rite of passage into manhood, and that is enough. 

By overcoming great obstacles and by facing danger alone, Jerry has acquired greater maturity and independence. In fact, telling his mother what he has accomplished could mitigate, or reduce, Jerry's personal pride in his newly acquired manhood. For one thing, she might display anxiety about his having tried such a dangerous feat, and then Jerry could feel some guilt about his actions. So, when he and his mother sit down to have lunch together right after she has "looked at him closely" and has seen that he is "strained," Jerry probably perceives the worry and anxiety in his mother's face. Feeling the need to relieve her anxiety, as well as having a child's need to tell his parent about an accomplishment, Jerry does reveal to his mother that he can now hold his breath for at least two or three minutes.

This revelation brings from his mother both praise and concern:

"Can you, darling?" she said. "Well, I shouldn't overdo it. I don't think you ought to swim any more today."

Jerry perceives from this response that he has made the right decision in choosing not to inform his mother of his feat. He wants his rite of passage to remain free of emotional and parental entanglements so that he can maintain his newfound independence.

In "Through the Tunnel," why doesn't Jerry tell his mom about swimming through the tunnel?

In the end, Jerry doesn't tell his mother about swimming through the tunnel because what he seems to want, at that point, is her approval and praise, and he likely knows that she would not approve of his decision to put his life in danger in order to do something so completely unnecessary and stupid.

In the beginning, Jerry understands that his mother worries about him.  "He was very familiar with that anxious, apologetic smile" of hers that signaled her twin worries: worrying about giving him too much freedom and worrying that she doesn't give him enough.  Therefore, in the end, to tell her that he had done something so dangerous would absolutely confirm that she has erred by allowing him too much independence.  He does not want to make her worry because he feels somewhat "chivalr[ous]" towards her and experiences the pangs of "contrition" when he thinks he's letting her down.

Further, he does want her approval and praise, and so he impulsively tells her that he can "'stay under water for two minutes -- three minutes, at least.'"  The narrator says that "It came bursting out of him" as some such news would from a little child.  This is an achievement that he feels safe telling her because it wouldn't endanger him or cause her to worry.  Like a child, he longs for her praise, and this is an achievement that she can (and does) praise.

In "Through the Tunnel," why does Jerry not tell his mother that he swam through the tunnel?

Jerry doesn't tell his mother of his accomplishment for two reasons. She is very protective of him. She treats him as a small child, even though he is eleven years old. She would be frightened and upset to learn that he swam through the tunnel. She might possibly watch him even more closely in the future. One theme in the story is Jerry's need to become more independent of his mother--to grow up.

Secondly, and perhaps more important, Jerry does not tell her because he does not need to tell her. Swimming through the tunnel is something he has done for himself to gain self-respect. He still wants or needs his mother's approval to some extent, however. At the end of the story, he does share with her how long he can now hold his breath.

By the conclusion of the story, Jerry has grown up in an important way. He has acted independently and proved to himself that he has discipline, strength and courage. Because this knowledge has now become a part of him, Jerry does not need to share it with his mother or prove it to the other boys.

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.