I'm sorry to hear that he's become so sick. In my Pearl's case, hers was more like a very bad case of chin acne and her lower lip would also become swollen. Prednisolone in pill form helped clear her up. All of my cats are still on a grain free diet but now they eat Merrick's Before Grain dry in both chicken a salmon flavors. They also eat some of the grain free Wellness canned as well. My vet didn't think that hers was caused by a food allergy but feeding her a grain free diet has helped reduce her breakouts. When I switched from Wellness Core dry to the Merrick's Before Grain, she did have a flare up. It has a lower protein content than both the Innova Evo which I used to feed them and the Wellness Core so maybe this was the cause. I treated her with pred to clear it up and now she's been fine for quite a while now. Maybe her body just had to get used to the new food. I've also read that it can be caused by fleas. I treat all of my cats with Advantage II every month so this hasn't ever been an issue. I hope that Oscar will feel better soon and that you'll be able to keep it under control. Good luck.