DS is 12. His equipment for school looks atrocious- he chews his ruler, his pencils, protractor, glue stick (not the glue thank goodness, the base), his pens, even chewing through two steel fountain pens this academic year 
I don't know how to stop him, he says he can't stop. Bit I am so worried about his teeth and jaw, and damn sick of needing to replace equipment (I send him with chewed stuff, but pens stop working when the end is bitten off, and he makes his pencils disintegrate!)
Suggestions please!