The Emerald Tablet — P.J. Hoover

Benjamin is different from other kids -- he can read minds and use telekinesis. But it isn't until he's sent to summer school on a hidden, underwater continent that he learns the truth. It turns out, Benjamin isn't really human at all -- and the powers he thought made him special, just make him normal. But then the mysterious Emerald Tablet chooses him as its champion and he's thrust into a mission to save the world.