Hi Geeko, so sorry to hear about your guys problem. First I must say your best bet is to take him to your vet and see if he can figure out what's going on as it could be parasites, or something we can't see.

Since I have no clue what may be the problem and can not personally help, I suggust maybe jumping over to the leopard gecko forum and asking them if they have any ideas that may help.

I sure hope your guy will perk up soon and all will be well again, please let us know how ya make out.
Leopard gecko forum

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PHEve / Eve