I am a Childrens social worker- and see this kind of thing more than you'd think. Usually, it signifies a problem, and the child may be drawing attention to himself- how are things at home and school? It may be useful to speak to an org called 'young minds' who will be able to point you in the rigt direction, or have a quiet word with te school nurse. It isnt taken as seriously as it should be, as there may well be a problem that isnt known about. If there isnt a problem he may well grow out of it. However, i would advise that if he manages to wear anything and not knaw te sleeves, to give lots of positive encouragement and praise for keeping his cothes nice, and mayb for him to help with washing- eg helping put it on radiators etc, so he can learn to treat clothes nicely- again lots of praise when this is done, and no telling off when he does chew away. Some kids think that negaitve attention is better than none at all, so that could be why hes doing it. Id also have a word to the gp to see if there is any probs- he could be chewing all kinds of fibres which may get trapped in his insides and could cause probs with his internal workings- your mate is right to be worried about this- good luck