I am sorry that you have been waiting for a response. I recently just logged onto the site and noted that your question hasn't yet been answered. I am a licensed veterinarian with over 25 years’ experience and would be happy work with you if you are still needing assistance.
After reading Oreo's history and treatment so far, your best bet is to continue working with the veterinary dermatologist that you have been so far. Until the MRSP is cleared up completely, you will not be able to evaluate any underlying skin conditions, especially related to food or environmentally triggered allergies. I suspect he has some degree of Atopic dermatitis
based on his history, but with the complication of MRSP, it will be difficult to try to manage that until the infection is cleared. Chronic ears can be a challenge, but the dermatologist would be the best one to manage those as well.
This is really not the best forum to seek help on a case as involved as his. The person offering advice has to be able to review his complete medical history as well as what medications and food trial were performed at any specific time and how he reacted to that. This person should also have an "in person" evaluation of his current lesions. That can not be offered in an online setting and would be a disservice. If you are having financial constraints, make sure to discuss that with the dermatologist and they will be able to help prioritize the best way to proceed.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.