Monster Max and the Marmalade Ghost by Robin Bennett
Max is an ordinary boy, except that he can turn into a monster when he burps... Max and Peregrine are volunteering at an old people's home when strange things start to happen. One resident is walking on the ceiling, another is riding their wheelchair through walls, and Reggie says his marmalade is haunted (no one listens). Can Max and his friends work out what's happening and protect his family and the local community? Things aren't looking good: the Marmalade Ghost is turning into a sticky Godzilla, Max falls out with his (joint) best friend, and then just when it can't get any worse, someone kidnaps his cat, Frankenstein... Time to 'Protect and Do Good Stuff!'