When it comes to neurological disorders, an MRI can be invaluable. There are many breeds prone to neurological problems and GSD happen to be one of them.
One of the most common d/o's that we see is called DM (degenerative myelopathy) which happens in aging GSD. It can occur alone, or along with other problems as well. There's a video here that shows mild-moderate DM case: https://www.youtube.com/watch?v=ukjJXs9C6D4 If you're unable to submit a video, it may help to look at this one and see if any of the symptoms are the same.
Along with an MRI, lab work being performed might reveal a clue as to the origin of the symptoms. There are vitamin deficiencies and diagnoses like tick borne diseases (example, rocky mountain spotted fever) that can cause circling, odd head positions, odd movements, etc. Before electing euthanasia, it would be worth looking into these avenues to be sure that it's not a blatantly obvious medical concern that's easily fixed. That being said, once a dog has progressed to neurological symptoms, it's often difficult for them to come back from. Many of them have lasting effects even if you're able to treat them. At 14 years of age, your boy is at a full life expectancy for a GSD and these complicated tests and even answers may only buy a short amount of time (and for that, a lot of money expended for that time). I'm sure your vet has advised that at some point the expense will probably be more than the benefit you or your boy will gain from the diagnostics. In many cases, we get no answer at all and the outcome is still the same.
If it's not already been discussed, but you do with to continue with diagnostics, a referral to an internist with a special interest in neuropathy/neurology would probably be your best shot at getting a definitive answer on what is causing this. If you need help finding one, reply back with your city, state and zip and I'll get to work finding the one most local to you.
Please let me know if your boyfriend has additional info or video.
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