Welcome..did the vet do any scraping or test for parasites? Right now..I wouldn't make any changes to his diet. Try and see if medication helps and if not if the vet can isolate the problem. I might use an oatmeal shampoo ..it's not that strong and is soothing to his skin. My vet had me leave the shampoo on for 10 minutes and rinse..rinse..and rinse some more. Good luck and keep us informed!.
Remember..some medications can cause loose stools so be prepared.