Well, chicken is a common allergen. You said you have not tried low-carb foods and wheat and corn are also common allergens. Many yogurts have fillers that are allergens. I'd get him on something with limited ingredients and a novel protein like pork, duck or fish and a carb like peas or millet. You can find these dog foods at specialty stores or Chewy.com has a good selection with free shipping. Fromm is a good and popular choice, perhaps a good place to start. There are lots of options. During this time I'd take him off of everything else, including the baths and supplements, if you can, so you can see how the new food is working. Give it at least six weeks before changing foods again.