Has your dog been tested for diabetes. Excessive thirst and hunger are both symptoms of diabetes.
Hi.. 6+ cups for a shepherd sound about right.. I've taken care of skinny sheps that need 7 cups a day..
I don't know much about Cerebellar Abiotrophy, but I do know that it is has symptoms similar to Degenerative Myelopathy, an incurable autoimmune disease much like MS to humans. DM is very very common among Shepherds(especially the old German bred lines), and can have effects on the kidneys(I forget what exactly), and produce a high ACTH level, which is what they measure to diagnose cushings.
Has he been tested to rule out DM? Why do they think CA over DM? I've never heard of a shepherd with CA, but have taken care of a handful with DM-wobbly back end, balance problems, etc are the main symptoms, and since it effects the kidneys and ACTH levels it may be contributing to his drinking problem.
Ask your vet about DM and get his ACTH checked.. If its high It may point to Cushings, but underlying I would suspect DM over CA..
Take care
Although he seems too young, excessive hunger and thirst are two of the first symptoms of "Cushings Disease".(After Kidney problems are ruled out). Just a thought.......Karla