I would still consider moving him in something very tight 15 quarts and see how it does, give him a few week and if he does better and eating behavior start picking up that will tell you that he just feel more comfortable in a more cramp environment, and if not than he could be one of those fussy eater.
They are all different and while I have add very young and small animals feeling secure in the largest enclosure I also have seen the opposite.
Pay attention and see how he behaves when you make the switch, sometime slight changes and your ability to observe will tell you a lot.
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