Youngest recognized prince of an islands/on the coast kingdom, but he and his mother live with family on mother side on one of the islands. Nobles usually have 4 wives, each with an area of expertise: diplomacy, politics, domestic affairs and military. And i think that other than the main wife, the rest are more of an advisory role and can be divorced or ask for a divorce to marry someone else as the main wife at any time. They usually travel by ships and the MC is expected to form his own commercial fleet in the future. When he travels to his father's palace, he sees a dragon and decides to become a dragon rider. Eventually he is given one with whom he's supposed to bond with over a long time. The other princes are in a contest to pick the heir to the kingdom. It eventually comes down to two of them, one of which pressures the MC to work under him. Also, i think the king is a couple of hundred years old, courtesy of bonding with a dragon, is bored of ruling a kingdom and wants to retire so he can go back to adventuring with a care. It's most likely on RRL, but it's been years since i read it.