Unfortunately he passed away this morning. There was slime and bubbles along the surface of the tank, and his bloat had decreased.
I believe some of his organs had been failing, as he seemed unable to digest anything at all (despite very small and limited feedings).
I can find some joy in knowing he passed peacefully during the night, with no added stress nor external injuries from the other fish. As soon as the ground thaws out I will give him a proper burial in my garden.
Thank you everybody for your suggestions and support, I really appreciate it