Monster Max and the bobble hat of forgetting - Robin Bennett

Max is just an ordinary, slightly scruffy boy, until he burps, when he turns into a huge monster, with teeth like swords, who can jump over buildings, roar and eat whole dustbins. It's great being a monster whose aim is to 'protect and do good stuff' until Max sneezes, when he turns back into a boy again, sometimes far from home and dressed only in his pants.
But something starts causing a lot of damage in his quiet suburb and it isn't Max. Trailed by his nemesis, Peregrine, a boy his age who has invented a POOP (portable operating omni prison) machine to catch the monster, Max has to find the real culprit before Peregrine catches him...