My Gym Teacher is an Alien Overlord

Luke is a comic mad eleven year old who shares a treehouse with his geeky older brother, Zack. Lukes only mistake is to go for a wee right at the wrong time. While he's gone, an alien gives his undeserving, never read a comic in his life brother superpowers and then tells him to save the universe. Luke is massively annoyed about this, but when Zack is kidnapped by his arch-nemesis, Luke and his friends have only five days to find him and save the world.