How can an otherwise smart and reasonable kid not be able to understand that this is a bad and undesirable habit?
It sounds like a sensory thing, as pps have noted. I would really encourage you to think about the issue in a different light. It probably isn't that he doesn't understand that it is not a good habit. I am sure that he does. It is that he is 6 years old and experiencing and incredibly intense NEED for this sensation. Imagine if you had an incredible itch that wouldn't go away. Everyone told you it that itching it was bad and you shouldn't do it, but it was the only way to get some relief. And you're 6 years old. That it what he is likely experiencing. Criticizing him for the chewing is probably not going to make it go away. It may, however, make him feel really rotten for needing to to it.
I would suggest a compromise: it is ok for him to chew on some agreed upon chewable items that you will provide. It is not acceptable for him to chew on other things. This gives him the sensory outlet that he needs without the destruction that is currently occurring.
Fwiw, I did not understand my son's sensory needs until he was a bit older than your son and I really regret it because he got the unintended message early on that these things were "bad" and that he was "bad" because he needed to do them. That message has been really hard to undo. I really encourage you to think about the problem in a different way and avoid, if possible, the negative feelings that your son might develop if his need isn't recognized. Best of luck!
