The Key of Kings by L.A. Griffin

When 13-year-old Paul Aderston goes on holiday with his parents, he does not suspect anything out of the ordinary will happen. Then, he meets a strange man named Dorian Lunarses who transports Paul to Alterra, a magical world. Before he knows what is happening, Paul faces the greatest challenge and adventure of his life. He meets powerful rulers, encounters bizarre creatures, and defeats sinister villains, all on a courageous quest to save the Key of Kings. Paul's unwavering bravery inspires the friends he makes on his journey, but can it save him from his enemies?