Boy on the Lake

When Jason skates on the lake, once everyone had gone, he sees/hears a little boy directly across from him. Keeping in mind that Jason is still a kid and is very lonely and insecure, this "ghost boy" seems to be an imaginary friend. Jason is constantly creating physical manifestations of his issues to make them easier to deal with--this is where Hangman, Unborn Twin, Maggot, and the ghost boy come from. Hangman personifies Jason's stammering, Unborn Twin and Maggot are his general social anxiety (always telling him not to screw up and the like) and the ghost boy was just keeping him company so he wouldn't have to be alone.

Creating manifestations to cope with difficulties is completely reasonable, especially for a someone of Jason's age.  Personally, I keep forgetting how young Jason is, which is incredibly odd given how important his age is to the story line. If he were older, he would be dealing with school, stammering, and "secret societies" differently. Instead of trying to break into social groups he would probably have the sense to just get through--that's what most kids in today's society do.