My 6-year-old daughter is also a chewer. Her teacher suggested letting her chew gum, and allows her to have it in school as long as it stays discreetly in her mouth and she disposes of it properly when she's done. We've been doing this about one month, and it does seem to be helping. Her nails and hair are growing unmolested, her clothes are staying in one piece, and her pencil erasers are staying more or less un-gnawed.
Is there a pattern to when he chews? In my daughter's case, it was happening at times when her active focus was somewhere else and she was sitting still - listening in circle time or reading time at school, watching TV, trying (unsuccessfully, usually) to pay attention in church, etc. Making her aware of when she was doing it and allowing her to have the gum at such times have helped a lot.
My daughter's teacher also told me that this sort of thing is not rare, and most kids do outgrow it by second grade or so.