Since he's 6, I am assuming he is now in school. Sometimes kids chew their clothes when they are stressed, worried or even having some difficulty with sensory processing. Try some other options such as: chewing sugar free gum, having something to fidget with like koosh balls, items with interesting textures, etc... Your school should have an Occupational Therapist that can give you some good ideas. I'm not saying your child has sensory processing issues, just that some children go through this as a developmental stage and he just needs some other ways to help him through this stage.
My dd also did the chewing when she was 5 - 7 and occasionally still does it at 9. I got some ideas from our schools occupational therapist and it was very helpful. Good luck!