He may be very dehydrated if he isn't eating or drinking. Misting is good but if he is sick it won't be enough.
He may need antibiotics for an infection or a vitamin shot to help him.
I think your best bet is an in person exam by a herp vet and treatment.
You might try offering some earthworms or slugs as well as some fresh fruit - maybe a bit of strawberry for example.
You can read here for more feeding hints
http://www.marietta.edu/~mcshaffd/boxt/faq.html#why
and here
http://boxturtlesite.org/faqturtle.html
But I really think at this point in person vet care is the best hope for him.
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