Hi, I do not have fat tails, but many other lizards, and when ever they refuse to eat and are losing weight/ getting thin it'a a good idea to have them treated as soon as possible for parasites/ worms.
Don't wait long as it sounds like your guy is goign down hill fast. The fact his tail has shriveled, is not good and he can also become dehydrated as well.
I'm sure others who actually have fatties can add something to this, but I would act fast like I said. Hope he hangs in there and perks up fo rya, let us know how he does.
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PHEve / Eve
