Could the frilly be impacted? Only a thought as if he isnt getting anything out, he wont put anything in. Try a warm water batch.
Also, my frilly had a similar problem when I got him but as soon as I introduced him to a bath, he drunk LOADS of water and then his appetite improved immediatly. Just thoughts.........
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AndyD

1 Mexican Kingsnake
3 Bearded Dragons
1 Egyptian Uromastyx