The first book in the My Brother is a Superhero series.
 
Luke is a comic-mad eleven-year old who shares a treehouse with his geeky older brother, Zack. Luke' s 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...