What's the Name of That Book??? discussion

I read this book around 2006 i want to say, but im not sure. From what i can remember it is about a boy is transported to another world where he is on a quest to defeat a dark overlord, i believe he meets a wizard who helps him. The villain is also in the boys home world as he travels back and forth. I can remember a part where the boy is being chased in his home world by the villain and says it sounds like wet slapping behind him like someone running in flippers. Later we find out the villain has webbed flipper like feet.

How old is the boy?

Is this book for kids, teens, or adults?