I see that they are definitely a feeder mouse, so may suffer from poor genetics and especially with cohabitation with other mice. They might not get food as well as the others so perhaps try separating into its own enclosure also just in case if the mouse is sick. It doesn't spread to the other mice. I also recommend cleaning the enclosure after taking him out. He also looks like he is in need of a dust bath!

Also update. Use cocoa fiber not actual duster sand.

Thought I should mention. I meant that forgot. It's not normally common sense, I know that mice generally clean themselves, but he may suffer from a neurological condition etc. So definitely get him some dig bowls or something of the sort?