It looks like a cyst, and maybe he's been licking at it and made it more irritated. The symptoms are most likely allergy related since you mentioned he also has an ear Infection, and he just had a food change. I wouldn't change his food anymore either, he does well on the prescription food, he tolerates it, and is symptom free when eating it. This is the food that Brutus does well on, so I would just continue to feed him the prescription food. If it ain't broke, don't fix it. I would switch him back, and just continue to clean his paw, you can use Hydrogen Peroxide, and put Polysporin on it at night, so there's less chance of him licking it off since he'll be sleeping. You can also soak it in either salt water, or Epsom salts and warm water, or even apple cider vinegar and warm water. I hope it heals soon, and he feels better.