Jenn, I had a really similar experience with George and I wonder if Calloway might be dealing with the same thing. George is one of my two 'BIG' cats and he too struggles to lose weight even when I restrict his calorie reduced kibbles. George is very sensitive with his whole back end and had a horrible tail abscess that required surgery when he was with his previous owner. He bites me as well if I touch him for too long on lower back or tail.
He's still big but not 'as big' as he was last year because he's getting more exercise in his outdoor run. But last year when he was really big, I noticed that he was trying to reach his back end to clean himself but he couldn't reach. I tried to help him with a wet cloth but he would have none of it. Each day I tried to help but it was getting worse. I finally was able to get a peek (he was very feisty about me looking down there) and then I saw that he had some poop caked on there and it was getting worse. I could see that his bum area was really irritated. I took him to the vet and because he is somewhat feral they had to put him under and shave his bum area. It was so red and raw and I felt terribly for him.
Once he was cleaned up, they sent him home with some Theraderm cream (anti-inflammatory steroidal cream) that I had to apply. They said it would make him more comfortable and it really did seem to help him. He was fine within a week or two and he is still able to clean his backend for the most part now.
Anyway, I wonder if it could just be a matter of irritation that has progressed due to his large size and not being able to attend to himself down there? I hope that is all it is.

My cat Stinky has had impacted anal glands in the past and when I went to pick her up she would kind of meow/scream which she never normally does. I saw her scooting her bum on the carpet and took her to the vet and it turned out both her anal glands were impacted. They 'squeezed' them and got rid of it. YUCK!!! She was great after that. But during the procedure she was very cranky and just about took the vet's head off. She hasn't had it happen since and that was a few years ago.

Stinky has also lost some hair around the edges of her ears in the past and it seemed like it was diet related. Once I switched her back to a food she was used to her ear hairs grew back.