Do you have any photos? It is hibernation time and this may be the issue. If so, this is a good thing. But if he is really emaciated it my be that he is on his way out. In this case providing proper lighting and getting some ants for him to feed on would be your best hope.
It takes some experience to know if his weight is where it should be for hibernation, so the photo is the best way to get a quick read on his condition.
There is a lot more to know regarding his general care. You can find much of this info as well as some photos that may help ID him at www.phrynosoma.org

I'm glad you took the time to research this and post.
Thanks
Mark
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www.phrynosoma.com