My Brother Is A Superhero by David Solomons

Luke is a comic-mad eleven-year-old who shares a treehouse with his geeky older brother, Zach. Luke’s only mistake is to need a wee right at the wrong moment. While he’s gone, an alien gives his undeserving, never-read-a-comic-in-his-life brother superpowers, then tells him to save the universe. Luke is massively annoyed about this, but when Zach is kidnapped by his arch-nemesis, Luke and his friends only have five days to find him and save the world…
Super-exciting, laugh-out-loud funny, and with enough heart to fill an entire galaxy, this is perfect for fans of Frank Cottrell Boyce. David Solomons is a meteoric new voice in children’s fiction.