What breakthroughs has Jerry acheived by the story's end?Consider: the tunnel, his feelings about himself, & his dependence on his mother.
By the end of the story Jerry has gone through a journey from childhood to manhood, symbolised most stridently in his journey through the tunnel.
At the beginning of the story we are introduced to a character who is on the cusp of adolescense, and very clearly feels responsible for his mother due to their enforced intimacy. Yet despite his feelings of responsibility towards his mother, he nonetheless feels drawn to the "wild beach", which is away from the "safe beach" and his mother's attentive care. The wild beach here can be said to symbolise independence and life away from the protection of a parent figure - note how Lessing describes the two beaches to draw out this comparison.
His discovery of the tunnel and the challenge that the French boys set him through swimming through the tunnel spur Jerry on to train hard and eventually succeed in his attempt to go through the tunnel. Although certainly at the beginning of the story it is Jerry's need to be accepted by the older group of French boys that drives his desire to go through the tunnel, it is interesting that at the end of the story he no longer feels this is the case, as he is happy to go back home and spend time with his mother. This indicates that the tunnel was more about a process of self-acceptance and doing something to show he could do it for himself rather than for any other reason.
His relationship with his mother likewise has changed by the end of the story. Jerry deliberately witholds his triumph, only relating his ability to hold his breath. The dramatic irony in his mother's response ("I wouldn't overdo it, dear") indicates the independence that Jerry has achieved in his journey through the tunnel - he has now entered an arena where he has secrets from his mother and is able to engage in activities, dangerous activies, away from his mother's protection.