What has Percival Wemys Madison forgotten?

Poor Percy Wemys Madison. He's one of the littluns the younger boys on the island, whose relative weakness makes them an easy target for sadists and psychopaths like Jack and Simon. Like the other littluns, Percy isn't just scared of those two vicious thugs, he's scared of pretty much everything—the Beast, the pig, the stormy weather that routinely ravages the island—you name it, he's scared of it.
In this Darwininan jungle, where only the strong survive, weaklings like Percy are largely abandoned and neglected by the island's self-appointed masters. All Percy can do is crawl into a shelter and cry, hoping against hope that somehow his terrifying ordeal will one day come to an end. As a desperate attempt at comforting himself, Percy repeats his name and address—Percival Wemys Madison, of the Vicarage, Harcourt St. Anthony—over and over again. It's like a magic incantation which Percy is using to try and ward off evil spirits. It's the only thing he has left to remind him of the civilized life he once led at home.
Yet when Percy meets the naval officer at the end of the story, he has completely forgotten his name and address. He's been so utterly traumatized by his experiences of life on the island that he no longer knows who he is. This incident is deeply symbolic: it epitomizes a collective loss of identity, showing us how the boys have been robbed of every last vestige of civilization during their descent into outright savagery.