Chocky by John Wyndham

From the back of the book:
’What does it look like?’ I prompted, a little sharply.He hesitated, and then drew a breath.’More than anything I’ve ever come across it resembles what our unscientific ancestors used to consider a case of "possession". They would have claimed that Chocky is a wandering, if not a wanton, spirit that has invaded Matthew.’
Polly, their daughter, had Piff, who, with her empty seat at the dining table and late-night demands for water, was a typical invisible childhood friend. But Matthew, their son, has Chocky. And Chocky seems to be well versed in higher mathematics, used to a thirty-two hour day and can offer a surprisingly coherent account of how a gravity-shielded, cosmic-radiation powered space-craft might be made to work ...
Who and what is Chocky, and what, on Earth, could it want with an eleven year-old boy?