Why not try Isaac Assimov's 'Nightfall'? Assimov is a great writer because his stories are short, and play on assumptions we make about our daily lives. 'Nightfall' for example is set on a planet with 5 suns, which only all set at once every 5000 years or something. Of course these people have never seen darkness or stars, and are terrified, go on a mad rampage, and destroy their civilisation. These sort of stories will improve your child's imagination, and teach him to question the world around him, and his perception of it.