I read this book as a kid in the 1970s. I don’t remember much, except that a teen boy wakes up in the morning and soon realizes that nobody can see him. He thinks he’s dead. He can’t really interact with the world, but he does manage to ride a bus without falling through the floorboards. It might be that he never existed–I seem to remember that people weren’t wondering where he was. It might have the word “disappear” in it.