he was very off balance, had a right head tilt, nystagmus (eyes twitching, kind of like he was dizze), and poor proprioception (when they flipped his hind paws over, he didn't immediately flip them back).
These are typical symptoms of Vestibular Disease. Also called, erroneously, Geriatric Vestibular Disease, it is not exclusively geriatric. Also easily confused with ear infection, epilepsy and/or stroke, depending on which symptoms present.
Google up and ask your Vet. Some are unfamiliar with GVD or don't think of it in a young dog. My sister's dog had this at age 12. As per Sis's Vet it is most often temporary, lasts a month to a month and a half and dog recovers with perhaps some mild retained symptoms. Typically recovery is as sudden as onset. Dogs with bad symptoms, like Laddie who could not walk at all, were often put down but now we know that after a rather extended time they mostly recover.
Bang on a month Laddie just stood up and walked again. He could not walk for a month and Sis put her back out carrying him outside to toilet. He retained some head tilt which lessened during his next four years of life. Yep
made it to 16.5 years old.
I am not diagnosing your dog over the internet but providing this a clue for you to investigate further. Good luck. If it is GVD, well, it could be much, much worse things.