Quote

"Maybe he understood for the first time that to the boy he was himself an alien. A being from a planet that no longer existed. " (McCarthy, 129-130)

The Man always tells The Boy stories, stories about his past, and about the world before the catastrophe that occurred. However, these stories may be too hard for the Boy to believe, because he never experience it, he grew up in the post apocalyptic world. To him the stories are just stories. The Man would then seem alien to the Boy, since he grew up in this alien world that the Boy knows nothing about. He is from a different world entirely almost.