Our beagle, Lucy had Cushings. This was when she was 12 years old and after reading about the treatment we decided it was too much for her at that age. She also had liver problems which the vets discovered when she had surgery for what turned out to be a ruptured gall bladder. She was eight years old at the time and ended up in doggy intensive care. We were told she wouldn't make it through the night but she was a tough and stubborn little girl and managed to pull through. Vets said the liver didn't look good and the numbers were off the scale, literally. Biopsy came back negative which was good news. But her liver numbers were high the rest of her life. We gave her four more years before she was diagnosed with Cushings. It left her back legs so weak that she had trouble walking and standing. Lucy developed pneumonia which was what finally took her.
Sorry I can't offer any information regarding treatment but just wanted to offer my sympathy and best wishes for Buddy. I did a Google search at the time Lucy was diagnosed and found several forums where canine Cushings was discussed. I found it very helpful since these were people who were facing the same decisions I was.