It's not all that uncommon. My son did it at that age and he has ADHD, but I think that it's also common for "normal" kids to do that, or chew on their sleeves, hair (girls) etc. I don't think it necessarily means he's anxious, although that could be it too. We did a sticker chart thing where he could get rewards if he came home without signs of chewing on his shirt. We also tried different options for manipulatives he could handle in school to keep his hands/mind busy, like little squeeze toys and other things that his teacher OKd.
There are even child chew necklaces you can buy - google search for chewable necklaces, chew tubes, things like that. Here's a couple of links. (and I wish I had known about these products myself back then!)
http://nationalautismresources.com/chewable-jewelry.html
http://www.amazon.com/Chewlery-ChewEase/dp/B001JHZQDY/ref...
http://www.amazon.com/AKU-SENSORY-RINGS/dp/B00178YC76/ref...