he might just have an air bubble in his belly and its making its way to the back end.lol. id put him in a tub with water thats low enough for him to keep his feet on the ground and stick it in the fridge. hopefully if its an airbubble it will come out sooner or later.
sometimes my adults swim to the bottom then float to the top and i see this as normal. their tails also float up too and this again i see as normal.