Sadly, Billy the Bird is no longer in print, but there is a good chance your library will have it since Dick King-Smith, author of Waterhorse and Babe, the Gallant Pig, is well known and prolific, many of his books coming in at under 100 pages. It is such a short and sweet story with a boy as the main character that I will review it anyway. If you can't find this book, please do seek out other Dick King-Smith, all of which involves great animal characters in one way or another.
Mary Bird is eight the summer she discovers her four-year-old brother Billy can fly under the light of a full moon. As Mary says, "he wasn't walking in his sleep, he was floating in it!" After consulting her guinea pig, Mr Keylock, and her cat Lilyleaf, both of whom can talk, Mary figures out when the next full moon will occur and their adventures begin. Billy is mistaken for a UFO and thwarts a cat burglar before a lunar eclipse ends his flying days. The cozy illustrations are a great addition to the story.
If your child likes Billy the Bird, suggest Flat Stanley.