I can understand that this would be disturbing. Could it be some type of OCD or tic? Or could it be pica? I think pica--repeated eating/chewing of non-food substances--often happens when there is a vitamin or mineral deficiency, especially for iron. Can you take the boy to the doctor for a full check-up and medical advice (and definitely include a blood test for iron)?

Good luck. I would be interested to hear what your son's doctor says, so I hope you will check back.