Noki has been eating Scooters left over wet food and we think he's allergic to something in it. He's licking the hair off his butt so he has to wear the donut. We know he's allergic to wheat but neither his dry food or Scooters wet food has any so we're guessing he's also allergic to the soy in the wet food. Trying to keep him from eating the leftovers and also switching it up with a different brand.
Also concur with feeding raw. Our cats never have problems, never need to get their teeth done, have the most luscious and soft coats, their poop doesn't stink, they're not at risk of kidney disease, etc., etc. We're lucky that we have a business nearby that specialises in raw feeding and makes premixes of whole wild and domestic animals meats, including organs (e.g. possum, wallaby, hare, rabbit, chicken, turkey, salmon, etc.) and small bones for cats. Even if you don't have that, you can make up your own. Just avoid large animals such as venison, pork, and especially beef. Beef is highly allergenic to cats (and dogs). Mimic a small wild cats natural diet as closely as possible - small animals, fish and birds with organs and bones. Feed a variety of meats over the course of a week so there aren't any nutritional deficiencies. Get hold of some green tripe if possible (not all cats will eat it but it's worth giving if they do).