I think it would be difficult to pick a place that was best for him without doing tests to find out what he is allergic to.
I noted that you feed him grain free foods. Is that for allergies and has it been proven he is allergic to grains? The reason I ask this is that they are finding that the grain free foods are causing cardiac issues in dogs. Food allergies are far less common than environmental allergies.
https://vetnutrition.tufts.edu/2018/11/dcm-update/