THere must be something about this age! My 5 year old has had a similar problem recently. It started with chewing on his shirt sleeve - which left holes, totally stretched them and ruined them. His teacher said there were a number of kids doing that. Then I noticed him putting lots of toys and things in his mouth.
Positive reinforcement really works for him. We use a jar full of marbles. Each day he did not put toys in his mouth, he got 1 marble to put in a vase (or smaller jar or bowl). When he didn't chew on clothes, he got 2 marbles. When the smaller vase is full, he gets to choose a reward. He does something with either my husband or me one on one and we trade off each time. He chooses a movie to go to, an old Star Wars movie to watch, lunch at McDonalds (a rare treat in our house!), swimming, etc.
It has worked really well for him. It took only a few days before he as done chewing/putting things in his mouth.