Under a silver moon - Anne Fine

In a hot, faraway land, two baby boys are born under the same silver moon: Haroun, the son of the sultan, and Akil, the son of the gardener. As children they play together until one day Haroun has to learn princely ways. He is waited on hand and foot and all he can do for himself is eat, so every day he eats more and more and grows bigger and bigger. No doctors can offer a cure. But one day a hooded stranger tells Haroun that if he digs the garden he will find a key that will give him good health. After weeks of hard toil, Haroun doesn't find the key, but he does lose weight, and finds health and happiness once more.