The Lost Prince by Frances Hodgson Burnett

A tale with the glamor of medieval romance, which centers about the mystery that five hundred years before Marco's day, the young prince of the kingdom of Samavia had walked away over the mountains, singing, and had never been heard from. And Marco, a lad of twelve, pledged to the future service of Samavia, lives an unpretentious, wandering life, trained in all worthy things by his father, til his journey as Bearer of the Sign is ended, and his appointed time has come.