I would use DIA-IONX because it helps to support the kidneys and adrenal glands. However I would follow your veterinarian's recommendation and correct electrolyte abnormalities and use the prednisone initially. You need to give the body time to respond. If the adrenal gland has been severely damaged it is necessary to provide steroid while the glands are trying to recover. You may want to try Drenatrophin by Standard Process to nourish the adrenal glands. I would also give probiotics and Omega 3,6,9 to improve digestion and reduce inflammation. I would also consider veterinary acupuncture.
Please keep us updated on your progress.