My dog just tested negative for Addison's. Now I have to put him to sleep. Please read and offer advice...

My dog was recently diagnosed with Cushings desease. We medicated him with Lysodren, which is like chemotherapy for dogs and deteriorated his adrenal gland. The opposite effect of Cushing's is Addison's. If the dog gets too much Lysodren he can get Addison's. All the signs were pointing to this... He went from drinking and eating too much to not eating and getting dehydrated, confused and wobbly. He just tested negative for Addison's. This means he could possibly have a brain tumor, heart disease or cancer. All of these tests are expensive, treatment is expensive and he probably won't live long. She suggests I put him down because as soon as he is off his IV for about 35 hours he gets dehydrated and sick. Any thoughts???