I'm starting the process of finding out if Arthur has an allergy causing his swelling issue.

[wnylib]

4. As a human who has several allergies myself,

and who once had a cat with autumn pollen allergies, I'd consider environmental possibilities besides food and litter. But, you have to start somewhere, so food and litter are good places to begin.

Is there a particular place that he likes to spend most of his time? A blanket or other fabric there that might be an irritant or allergen? Someplace that he sits a lot since it's his butt that's affected? An allergy to some kind of house mite? All homes have them, no matter how clean, and people can get allergies to them so maybe cats can, too.

Also, has the vet x-rayed the area for anything in Arthur's intestines or anus that could be causing the problem?

Poor guy. And poor you. Good luck to both of you on tracking this down.