Is this a symptom of not enough pred? or does it have to do with cortisol levels?
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I think it is 2 sides of the same coin. The treatment for adrenal insufficiency is a synthetic corticosteroid to replace the less than adequate level of natural cortisol. Adrenal insufficiency is probably the single biggest reason why it is so difficult to successfully taper off prednisone. No matter how slowly we taper, getting completely off Prednisone is easier said than done, particularly for people on long-term treatment.

If he currently has no pain on 1/2 mg of Prednisone every other day then it probably isn't PMR. When I tapered off Prednisone, it took at least a year for the overwhelming fatigue to improve.

The weakness I still have is currently confined to my lower extremities. My legs are weak because of severe lumbar spinal stenosis which went undiagnosed when I was on prednisone. Prednisone controlled the pain that you would typically experience from spinal stenosis but Prednisone didn't prevent the damage caused by chronic inflammation.

Spinal stenosis is often caused by age-related wear and tear, such as thickened ligaments or bone spurs that narrow the spinal canal. Prednisone cannot reverse these structural problems. It only treats the symptoms (inflammation) rather than the root cause.