Wow, is he really 15! I was sure he was just a puppy not that long ago.
I've been seeing a specialist with Della because she's having cognitive issues (she's been diagnosed with a slow growing brain tumour).
I had gone through all the same thoughts as you. Specialist said a stroke is pretty noticeable. WIth cognitive decline/dementia dogs lose their perception of themselves in space and it starts slowly - they start getting stuck under things, maybe go to the wrong side of the door, miss a step or just face the wrong way. Circling is a pretty late thing. Gosh I remember when Tully would get stuck in corners and cry for me to get her, it was heartbreaking. They also lose their problem solving skills.
Circling and loss of proprioception can be neurological. Della started with large, barely noticeable circles - she'd always go off sniffing one direction. Nowadays she just circles around me (very annoying) always to the left. I have to snap her out of it all the time. She did have a grand mal seizure and the start but specialist thinks when she stares vacantly that could be a seizure too.
Could he have hurt his hock? I remember when Tully did hers the orthvet said shelties have weak hock ligaments.