I don't see the "today" photo. However, it sounds like he may have a bacterial infection.
In the "healthy" photo, his caudal (tail) fin looks a bit ragged and dark. This can indicate fin rot.
How large is the tank, and how often do you change the water?
I would start by doing a series of large, partial water changes. This will remove any ammonia, toxins and bacteria in the water.
Then, I would try using a gram-negative antibiotic. My choice for fin issues is the furan group, which includes any of these meds: API Furan 2, Hikari Bifuran, or Jungle Fungus Clear.