Senior Sheltie is seeing a lot of hair loss

My 15yr 3mo old Sheltie “Dexter” started loosing his hair about 2 yrs ago. I first noticed it around his neck, about where his collar use to ride. Thinking it was irritating him, I took off his collar and let it go for a while… Some time later I had him into the vets and asked about the hair loss and the Dr. started doing blood tests. We looked at his Thyroid, tested him for Cushings Disease, has gotten an MRI and have tried multiple medications to try to combat the hair loss to no avail. As of today he has loss most of the hair around his neck, is loosing it off his hinds and the area where they shaved for his MRI has not even begun to grow back. I love him so much and I know he doesn’t have a lot of time on this earth left with me and don’t want to subject him to the poking and prodding of the Vets, where he’s terrified of going… Thinking of switching him to a grain / gluten free diet and thought I’d reach out to the Sheltie community for any advise they could offer. Thanks in advance!

Ray