Yeast infections are often a symptom of atopic dermatitis. In other words it may have absolutely nothing to do with his diet.
Ask your vet for a referral to a veterinary dermatologist for testing and an accurate diagnosis.
Your regular vet has no choice but to treat the symptoms and hope that they will go away for a while. It is very important to prevent infection, only antibiotics and other medications can do that once the skin condition is out of control.
You seem to ask the same questions over and over again, maybe you are expecting different answers? No one here has examined your dog?
Anyway, you can review my previous responses, I hope that you will find something helpful.
Good luck
