Chewing seems to be a common AS stim. My son's shirts used to get destroyed like you wouldn't believe. But, some things time does help with. He would rather have a carrot now, although pencils still take a beating, his shirts are mostly left alone.
Unfortunately, while a stim may be within his control, it IS something he needs to do to self-calm. You can encourage redirection, but try not to make an issue of it. Straws are good, if you are looking for a replacement.
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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).