Thanks for the answers to my questions. The fact that he's not pacing and is not panting as much is a very good thing.
The symptoms that you describe are actually quite common ones for older dogs and can be secondary to several different conditions.
1. He could have had pain/discomfort somewhere in his body...hips, knees, spine.
Since he's already taking Metacam, you won't be able to safely give him any additional over the counter drugs such as Aspirin or Tylenol if he starts to pace again; this would basically be double dosing him and might cause gastric ulceration.
He may need pain medication although this would have to be dispensed by a vet.
2. Senility or cognitive dysfunction. These dogs become confused, can experience memory problems, can become very restless at night where they can't seem to settle down (Sundown Syndrome similar to human); they often pant but can whine as well. The signs can be intermittent but tend to progress with time (although the rate at which they progress is different for every dog).
There are some treatment options for this condition (which primarily involve supplements). Since he's not your dog, you probably don't want to consider their use, especially since many of them take time to build up in the system and won't be particularly helpful to you in the short term.
But should you want to know what they are, let me know and I'll provide a list.
3. When older dogs start behaving in very unusual or strange ways, I always worry that they may have a brain mass, I’m sorry to say. Usually there are other neurologic symptoms, though. It would take an MRI to diagnose this problem but most brain masses are not amenable to surgery.
4. Heart/lung disease.Heartworm disease, congestive heart failure, lung cancer (I’m sorry to say), pneumonia, etc.are all possible causes especially if there is exercise intolerance (the dogs are very tired after a short amount of exercise), weight loss, or coughing or significant lethargy, etc.
From your description, he doesn't sound as if he has one of these problems, but I mention them to be complete.
5.Aging changes are commonly seen in older dogs such that their lungs become less elastic, more fibrotic. This basically means that they don't oxygenate well so they pant more.
His current panting (absent the pacing) could simply reflect this and would not be concerning.
Since he currently appears more comfortable, I'd continue to monitor him. If he starts to pace again and you have access to Tramadol or Gabapentin, I can provide the dose if I know his weight.
I hope this helps; again, my apologies for the delayed reply. Deb