If he's eating, and the tail is losing mass, maybe he has a parasite? That is the only thing that makes sense to me. Maybe somebody else has more insight to this issue.
Do you know how much weight he's lost? Do you have a way of keeping track? If not you may want to invest in a scale - I got a "pocket scale" on ebay for less thant $15 with shipping and it works great. I think it was originially intended for weighing marijuana, as the seller also sold other interesting paraphanalia. lol. But if I tare a bowl, it is really easy to weigh my babies to make sure they're gaining and/or maintaining.
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-Kristin
0.2.2 Leopard Geckos
(Kumquat, Tamale, Jujube, and Nougat)