Put all things you don't want chewed out of his reach ... oh hang on, he's a cat and has access to everything in the house! Sorry, a lame attempt at dog humour ...

Mini is a bit like that, but he keeps his chewing to cardboard boxes, my clothes and library books with an interesting smell. Are you able to restrict him to only part of the house when you're not able to watch him? Does he like chewy things like cardboard or toy mice to try distracting him? God, that's a tough one!