We need to know your specifics of your husbandry. If something is wrong with the husbandry it could be affecting appetite.
-how old/big is your dragon?
-what size tank?
-what are the temps and how are they measured?
-what substrate?
-any cagemates?
we also need to know a little history...
-when did you notice your dragon wasn't eating as much?
-is he still defecating?
-how is he acting otherwise? is he still bright and alert or is he lethargic?
- have their been any changes in your dragons life (cage placement, temps, cage set up, increased traffic in the area of his cage?)
If your husbandry is correct i would get a fecal sample done at the vet to look for parasites. that would be my first suspect...there isnt much else i can guess at without knowing more about the situation. If your dragon is noticeably losing weight you should see a quality herp vet asap, before its too late, many people wait too long to bring their herps to the vet and they die in the process. if he is still defecating, bring a sample with you.
until then you can keep hydration up with several long warm water soaks a day, and you can try giving him gatorade or pedialyte with a dropper as well. These are not cures however, just methods to hold him over until his vet appointment.
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0.1 Bearded dragon (Hannabil)
0.1 mexican Black kingsnake (Morticia)
1.1.2 Leopard Gecko's (Pogo, Louise, Orion, Jeffrey Nothing)
0.0.1 Tokay Gecko
1.0 Blue Tongue Skink
0.0.1 Reverse Okeetee Corn (Autumn)
0.1 Bullmastiff (Asha)
4.1 Cats (Poe, Tucker, Abhid, Felicity, Emmy)