John Wyndham 
Chocky [EPUB ebook] 

David Gore becomes concerned that his twelve-year-old son, Matthew, is too old to have an imaginary friend. His concerns deepen as Matthew becomes increasingly distressed and blames it on arguments with this unseen companion, whom he calls 'Chocky’. As the story unfolds, it becomes clear that the friend is far from imaginary, but is an alien consciousness communicating with Matthew’s mind—a fact that is of interest to shadowy government forces.