Hair loss

My doxie is 7 years old. He has lost his hair on his underside for the second time. Took him to the vet where they did a $500.00 blood text, what a joke, that determined nothing. He eats high end food, grain free, egg, chicken, minimal fresh fish, chesse. Keep in mind these are very small treats. He also gets a lamb and rice mixture with his kibble. I changed his shampoo to an oatmeal based product. Any ideas what I can do to help him reclaim his fur? His manscape is not terribly attractive but we love the heck out of this little guy. Help!