My DS7--who is HG+ and definitely not an AS kid--quit sucking his thumb immediately when the dentist told him to stop. He was 3 or 3.5yo at the time, and he literally never put his thumb in his mouth again after the dentist said to stop.

I don't know that this means anything in your son's case, but I think it might indicate that such a behavior isn't necessarily AS-related.

Granted, we have engineers on both sides of the family, so it's possible that we're all semi-near that end of the spectrum. But no one would look at DS7 and ID him as having AS. No one.

Don't know if that helps at all...

Kriston