Eric's advice is based on real experience and well worth following. But first things first: I found that even 75 mcg Fentanyl patches needed to be changed every other day and never gave me pain relief for the prescribed 72 hours. My doctors agreed on only going 48 hours between changes plus they knew I boosted the opiates with a glass of wine at night and I did take hot showers. Individuals vary in their responses and don't fit neatly into those little package insert pigeonholes.
My recommendation is to talk with his doctors about this problem. Oh, and yes for many of us the pain was excruciating enough that even opiates only dulled it. I supplemented my pain control with medical MJ and that did the trick.
Charm