Bedding
How much bedding does he have? Does the light from the window shine in his house? He is maybe trying to block out light?
Oh and he shouldn't eat the cardboard box - not that it would harm him if he did - I assume you give him empty toilet roll tubes to chew on already - if not, he will appreciate them I am sure - but you want the box to be bigger than the cage base to catch any fallout.
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