Out of This World - Chris Wooding

If he had it his way, Jack would spend his days drawing aliens and epic space battles. But it seems Jack's daydreams of action and adventure will remain just that: daydreams.
Until his 12th birthday, that is. That's when Thomas, the annoying kid from school, stumbles upon a weird machine that sends a beam of light into the sky. Suddenly, Jack's world is turned upside down when intergalactic beings appear on Earth and try to kill him. His only hope for survival is a ragtag team of alien bounty hunters: Boston Sark, Mazzy, Dunk, and Ilara.
Through his new companions, Jack learns of the mysterious Gradius Clench, a precocious superspy and the universe's most wanted kid. And he looks just like Jack! Now, the race is on to find this elusive Gradius Clench - through fantastic landscapes, seedy pirate hideaways, and worlds Jack could only imagine - and maybe, just maybe, figure out how Jack fits into all of this...