I always wondered if he was mentioned by name and then never mentioned again to suggest that he was "lost." The bigger boys didn't take much specific care to account for all of the little ones and perhaps he wandered away never to be seen, or even remembered, again. There is no textual evidence for this -- only a theory.
As students of literature, we would like to think that the details matter in a piece of writing. In this case, it does seem odd that so few of the boys are actually given names, and Harold is one who has a name but no particular role in the story. Like my colleagues, I cannot really explain that, but there it is. Add it to the unsatisfying explanations of how, exactly, the boys landed safely on the island while the pilot and plane are apparently at the bottom of the ocean. Sometimes things are just inexplicable.
It is rather curious that the choir boy named Harold is not mentioned after his introduction in Chapter One, for the others--bill, Robert, and Henry--have roles in the plot. Henry, of course, is little boy who sits on the beach and is the target of Roger's stones thrown just around him, indicating that civilization still has a hold upon Roger's arm and sadistic nature. Perhaps, if there is any significance to the boy Harold, it is in his name as it may presage loss, although there is little, if any, support for this supposition.
Harold is a very, very minor character in this book. In fact, his name is only mentioned once and we never hear from him again.
In Chapter 1, Jack Merridew lines his choirboys up when they come out of the forest in response to the conch blowing. The choirboys give their names. There is Maurice and Roger, "Bill, Robert, Harold and Henry."
This is the only time that this character is mentioned by name in the book.