My son is a chewer. Shirts. Pencils.
It gets expensive, lol.
He no longer chews his shirts, but I say he IS a chewer because the need for the stim is still there, even if he's learned to apply it more appropriately.
You have to teach them to divert the need to chew to something else. Straws are a good one. Or carrots. And to encourage the diversion you can put him in short sleeve shirts (although he'll go for the collar).
It is also helpful to discuss the habit, and then develop signals to let him know when he is doing it and needs to stop. Something gentle and unobtrusive, but that he understands.
Many ASD kids chew their own hands, which can get a bit scary, depending on how far they go with it, so be wary of inadvertently sending him to an alternative you find less preferable (if he's a light chewer the hand is fine, and some people think perfect, but I just could never see that one for my child).
As with all stims, it tends to be related to stress and a desire to self calm. As you mitigate the causes of stress in his life, you also mitigate the stim.
Do create a fun chew festival he can freely pursue at home. Keeping stims in check at school and public takes a lot of energy; at home they should be free to be themselves. Without ruining all their clothes and giving themselves rashes
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).