Paraneoplastic Syndrome: Diabetes Incipidus - 1266338

This is a complex question and I apologize for the length!

The symptom that caused my husband's stage IV lung cancer to be discovered was thirst. No amount of liquid quenched his thirst and he was urinating constantly. Also, starting at 5pm he would get what we called "The Dementors": attacks of extreme anguish/anxiety/despair/discomfort...so extreme he would cry out, clutch his head & writhe on the floor. He reported that during those episodes it hurt to exist.

After 2 months of tests his primary care doc sent him to an endocrinologist. The endocrinologist found that my husband's pituitary was not producing antidiuretic hormone (vasopressin), diagnosed him with Diabetes Incipidus, and prescribed Desmopressin. A month later he sent my husband for an MRI which found the brain tumors (more than 25l) and the subsequent CT scan found underlying lung cancer. (Time from first doc visit to dx: 3.5 months)

In the beginning, the thirst symptoms of the DI were 90% managed by the desmopressin, but at the end of every 12hr pill cycle he still gets thirsty. The Dementors eased at first, but got worse until the cancer was found & he was put on 2mg/day of Dexamethasone.

After 6 rounds of carboplatin/alimta the PET scan last week showed a 2cm tumor left in his lung, they couldn't say whether it was active. (in May at dx, the tumor was 9cm, multiple spots in both lungs, 4 solid lymph nodes, extensive bone involvement & a complete cancer-caused fracture of his femur). The brain mets are gone.

We hoped that after weaning him from the Dex, The DI and The Dementors would be gone. They are not. In fact, 2 weeks after tapering the Dex the symptoms are back worse than ever. The oncologist doesn't have any solutions for us other than going back on steroids which we want to stop bc of other side effects. Should we find an endocrinologist? A neurologist? Any ideas?

Thank you!