Does he chew drinking straws? I'm trying to picture what he's chewing on that's plastic.
A LOT of kids chew drinking straws, if that's what he's doing. Chewing helps organize and calm the brain - does he chew during times when he has to concentrate, like math?
Is your son taking meds for the ADHD? ..Have you spoken to your Doctor ?
Question is how do u cure this? May need medical intervention!
Have you ever watched My Strange Addiction on TLC? Well on one of the episodes a 16 year old girl eats plastic. She does this as a comfort thing. Like some people eat comfort food, she ate plastic. When did he start eating plastic? Did something dramatic happen before he started this habit like a death, a new family member?
But If I were you I would def watch that episode of my strange addiction, they have them available for online viewing. I think this will help you a little bit.