Probably pretty slim. If you can put a shallow bowl and let him soak in warm water it may help if dehydration is part of the problem. A shallow clay dish like you would put under a clay plant pot would work well.
If he won't start eating on his own it is pretty hopeless.
What are you feeding him?
Sometimes winged insects such as moths can help stimulate a feeding response. You can catch them easily enough by a porch light.
Good luck!
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