It isn't ever going to be a steady progression.
Patience.
Interesting comments on the chewing. How bad does he hurt himself? Some parents think of it as a good stim, in that the hand is always there, so self-comfort is right around the corner. But I guess that depends on bad he harms himself. I know another boy who does that, and he'll bleed, he's got scars, but nothing too serious. His parents just let it be, in a way; they don't try to stop the behavior, but they do look to gently redirect and also work on finding/mitigating the triggers.
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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).