Game Changer by Neal Shusterman
All it takes is one hit on the football field, and suddenly Ash's life doesn't look quite the way he remembers it. Impossible though it seems, he's been hit into another dimension--and keeps on bouncing through worlds that are almost-but-not-really his own. The changes start small, but they quickly spiral out of control as Ash slides into universes where he has everything he's ever wanted, universes where society is stuck in the past . . . universes where he finds himself looking at life through entirely different eyes.