Feline Acne Treatments? Please Help if You Can.

I noticed my cat suddenly broke out on his chin, I've seen pictures of how bad it can get from scratching at it and he's been scratching pretty badly at it and his fur seems "dirty" and stained from it. I'd take him to the vet but they'd just do a bunch of biopsy tests and skin scrapings just to tell me what I already know. I noticed they sell cat wipes at petsmart but not sure if any have the chemical ingredients that apparently help with acne. (benzol peroxide, chlorhexidine, or providone-iodine.) 

My ideas for possible causes are:

Recently he decided that he only wants to eat limited ingredient fish. I buy him the nice stuff, real pieces of fish with all essential vitamins, nice, expensive food. However, he DOES sneak my roommate's cat's food which is horrible because she feeds her absurdly unhealthy cat nothing but dry meowmix food out of a plastic bowl.

He might have allergies to her plastic bowl or the dyes on the bowl or the dyes in the food. It might also have an allergy to grain because he seems to vomit when he eats bad quality food. I don't know if that would cause acne though.

I personally believe it is the wet food getting on his chin. Especially an oily food such as fish. I am very willing to clean his chin after every meal, I just need a good/safe product to use that is also effective. 

Our vet didn't do any scrapings or tests... maybe she was just familiar with the look and location, especially considering how common chin acne is for cats.

If the acne is mild and doesn't look infected, our vet suggested using human grade acne wipes (just don't wipe near his upper lip or close to his mouth). Sometimes these have a strong scent and leave a weird feeling - on human skin, at least - so some cats don't tolerate it well. Just to keep it clean, you can dilute some hydrogen peroxide with water. Some acne products with benzoyl or salicylic acid sting on broken skin, whereas hydrogen peroxide doesn't sting but it may just bubble or fizz.

If there's a way to keep him out of the plastic food bowl, that would also help... either by asking or helping your roommate transition to a different type of dish or just keeping him out of wherever the other cat is fed. The porousness of the plastic may be exacerbating it. If it's infected, a vet visit may be in order just to be on the proactive side.

Sounds like an allergy to me. My cat is allergic to most foods and especially grains. He gets acne all over his chin and scratches miserably!

Thank you for your help. Unfortunately my roommate is an older woman, my friend's mother. She isn't right in the head and is impossible to communicate with since she's deaf from brain damage. She's even fed my cat bad food in front of me as I was telling her about his allergies.

I plan on moving sometime next year if I can afford it. So I'll have time to undo damage then, I just want to keep him as healthy as possible right now