The Amethyst Tower (The Mer-Prince Adventures)

A boy of land and sea. Cursed to spend his days as a merboy and his nights as human, twelve-year-old Thomas finds himself stuck between two worlds. In order to break Rillana's spell and return home, he must learn the art of life magic, but merfolk law forbids it. His only choice is to learn magic while he's a human. In the nearby kingdom of Treslar, Thomas meets a strange teacher with a connection to Thomas's past. But if Thomas can't stay out of direct sunlight, he risks revealing his most carefully guarded secret.