Interworld by Neil Gaiman

An average high-school student, Joey,  who is shunned by his school mates finds himself at the center of an inter-dimensional attempt to take over every instance of Earth.  Turns out that he has some talent for foiling these beings using a simple formula of omega over infinity.  With his capabilities he does more than just stop the beings and rescues not only the Earth, but the people trying to save all the Earths.