How old is he? If he's an adult a 20 gal cage is rather small, adults should be in a 2'x4' cage at least. Unless the Indonesian subspecies is alot smaller than Northern BTS's.

What kind of food have you offered and how much? He may be more inclined to eat some superworms or silkworms (I would lightly dust these with calcium powder since he is suffering from MBD). Have you brought him to a vet since he came into your care? If his MBD is well advanced, he will need medical aid to get back on track.
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PHLdyPayne