The Ring of the Slave Prince — Bjarne Reuter

Fourteen-year-old Tom O'Connor, a poor, adventurous, charming liar who lives with his mother and half-sister at a tavern on the island of Nevis in 1639, rescues a slave from drowning, learns he is a prince, loses him, travels the Southern Hemisphere in search of him, and finally brings him home to Cape Verde, hoping for a grand reward.