Have the vets been able to determine what is causing his skin problem? He may need to see a specialist. He could be having a deeper problem which is causing his skin to act up and treating the skin is only relieving the symptoms, not the cause. Meds, baths, and lotions will only give temporary relief for the skin, but it's not going to cure the entire problem. I read that hypothyroidism can cause serious skin problems as you described.