Good Morning and welcome to Just answer. I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
I am sorry that Zac is not feeling well...
Most likely Zac has suffered an injury to a disc in his neck. This is very common in older german shepherds because of their conformation. Since he does not have any digestive issues, this is the most likely cause for his pain and lameness. Since this is deep within the vertebrae, he will not show pain when you touch or palpate these areas. This can happen when Zac jumps up or down on something or just moves the wrong way. The best thing you can do for now as long as Zac is eating well now, is to give him aspirin at 5 mg per pound every 12 hrs. This will decrease the inflammation that is causing his pain and start to stabilize this disc. Do not allow him to jump on or off anything or even climb stairs to allow this disc to stabilize. It is best to have your vet examine Zac to be sure that this is the case and that this treatment is appropriate. This is not likley to be OCD since this is seen in younger dogs.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please Reply so I can clarify for you
Sincerely, Dr. Loretta