I only had my tat for a few months or so until he died while I was moving residences.
Do the other fish in his tank eat prepared foods, or just live? Try to get him on krill or smaller market shrimp fast. make sure the size of the shrimp isn't intimidating to him. Tatuaia aren't nearly as ambitious as armatus, hence why they are sometimes more difficult to get off live.
My tat always seemed skinny, and part of it just has to do with body type. they definitely aren't as "stout" as armatus.
I loved my tat. Try to provide some decent cover that doesn't allow him to completely hide. Driftwood laying on the ground, or rocks that butt up against the back of the tank, but allow for some hiding spaces will be appreciated by your fish, and will make him more comfortable.
As Trev has said in the past, never use any medications for injuries/infections, just salt.
Good Luck.