For some reason some kids this age want to chew, my 6yr son chews his sleeve and i see them in his class doing it! I tried giving him something else to chew on so he didnt ruin his clothes, it was a piece of ribbon and it did work until he went to school with it and they took it off him!
If i tell him to stop he does it just as much so now if i see him doing i take it out if his mouth without saying anything, I dont think he even realises he's doing it most if the time and this seems to be working so far.
I actually remember being shouted at by a teacher at infants school for chewing my pig tails, she said she would cut them off, how horrible they were back then!!