Interestingly enough, William Golding never reveals Ralph's last name in Lord of the Flies. In fact, the reader does not know most of the boys' last names in the novel, and there could be many reasons behind Golding's decision to exclude that particular detail. In many ways, the boys' surnames represent civilization and a connection to their families and parents; once the boys are on the island, that connection has been severed, and the last names are ignored.
There are two surnames of boys that the reader does know: Jack Merridew and Percival Wemys Madison. Jack's last name is introduced at the very beginning of the novel; Simon refers to him as 'Merridew' when he asks if they might sit down. Simon's use of Jack's last name could be taken as a sign of respect for the head choir boy. The other boy, Percival Wemys Madison, uses his last name and address as an "incantation" to make himself feel safer (94).