Poor Calloway. I sure hope that he'll start feeling better soon. I always thought that a hot spot was caused when an animal kept scratching that area or rubbing on it so it then became raw. Maybe he's been scooting his bottom along the floor and this caused it. I'm sure that by shaving this area it will help to keep it cleaner.

As for a new diet, you might to want to consider putting him on a high protein diet like Innova Evo or something similar. Since I've had all of my cats on it, they've all lost some weight. I thought that my Ziggy was going to become a very overweight cat but the Innova Evo even helped her to lose some weight. I don't free feed though so I'm sure that this also helps. Finae might also be allergic to grains and the Innova Evo is a grain free food so this might also help her. Good luck.