Hom — Jeanne Willis

When a boy washes up on a desert island, he is sure he's on his own in the world. 

But there's someone else living there: Hom, a peace-loving creature who has lost his family, too. Alone on the island together, they learn from each other and become the best of friends. So when a rescue ship appears on the horizon, the boy has a big decision to make . . .