It actually suggests to me that he is overwhelmed by such a large enclosure, as he was likely housed in a six quart tub at the breeder's.
I would suggest that you move him to a smaller tub until he is eating consistently for you - then if you must use a tank, a 10 gallon for now. A 20 gallon is much too large for a wee baby - and he's telling you so by his behavior and refusal to feed.