Split (1974)

"A boy entranced."

When his father runs off and his mother hooks up with a mustachioed new beau, eleven-year-old Mark becomes convinced that an uninvited talkative space alien, going by the name "Split," has taken up telepathic residence in his mind, and is leading him to question the norms of planet Earth. Only one teacher is willing to entertain the possibility that Mark just might be telling the truth.