Good Morning and welcome! I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
I am so sorry that Barney is not feeling well.
Most likely Barney has suffered an injury to a disc in his back or has arthritis that has been aggravated. This is very common in older labs because of their conformation. Since Barney does not have any digestive issues, this is the most likely cause for his pain which is shown by not being able to get comfortable. Since this is deep within the vertebrae, he will not show pain when you touch or palpate these areas. This can happen when he jumps up or down on something or just moves the wrong way. The best thing you can do for now as long as Barney is eating, 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. I would also recommend that you give Barney a daily supplement of Glucosamine/Chondroitin available at pet store and will add lubrication to his joints to help make him more comfortable. It is best to have your vet examine him to be sure that this is the case and that this treatment is appropriate.
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