mroades, I doubt yogurt would help as the dog’s yeast infection isn’t systemic; it’s limited to (presumably) the outer ear. You need to clean the ear on a very frequent basis to help keep the issue at bay; a product like Epi-Otic is readily available online (I used to buy it from Amazon.com) and has salicylic acid in it to help keep the ear dry (yeast loves a moist environment).
One thing that’s so frustrating about food allergies is there really isn’t a test for them. There is a blood test available, we were advised we would basically be wasting our $$ by doing that as it is not reliable for environmental or food sensitivities.
Instead, my parents’ Lab was skin tested for 70+ environmental allergens (including cats & horses!). Interestingly, he was moderately positive for Johnson grass and reacted severely to wool - and my parents don’t have any wool in the house! Those were the only two allergens he reacted to (well, he reacted to the positive control… but he was supposed to 
The dermatologist also told us there really isn’t any test (skin or otherwise) for food sensitivities - you have to use a trial and error approach which can be frustrating!
We were also advised that grain free diets will not help in the case of a true food allergy as they are not novel proteins. They do contain chicken, beef, etc…
Do you know if the vet has ever taken an ear swab and sent out a culture & sensitivity? There may be a bacterial infection along with the yeast (although he should be able to see bacteria on a slide if he looks at a swab under a microscope).
Best of luck - ear issues are no fun for dog or owner…