If he is acting that lethargic then i would say he is too far gone to do any home treatments and he really needs to see the vet. I would suspect parasites. How long has he been on the decline? If he is very thin he must have been getting worse for awhile, when did he stop eating?
A few things, its important to know the specifics of your lizard and his care:
How old is he?
Where did you get him?
what size tank is he in?
what are the temps (high and low)?
Does he have UVB light?
what do you supplement with?
What is the substrate?
What do you feed?
Is he housed alone or with other BDs?
Call the vet and make an appointment, explain to them how dire the situation is and they should get you in right away. Until then you can try giving him some pedialyte or diluted gatorade with an eyedropper to hyrdate him. You can also try some soaks in very shallow warm water, watch him and make sure his head stays above water. If he is housed with other lizards seperate him and keep him warm (basking spot of 100-110, general temps in the high 80's) You can try giving him some slurry of chicken baby food with vitimins and calcium mixed in with an eyedropper as well, but he really needs to see a good reptile vet asap.
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