Fentanyl is roughly 100x stronger than heroin as an opiate goes and is the strongest pain killer available to us (Carbofentanyl is stronger but used for large animals) and 50mcg/hr is a healthy dose of that drug.

There are a few things to consider however before upping the dosage again, the first is the type of pain he's feeling. If he's experiencing nerve pain (neuropathy) then an opiate pain medication won't help. As many people have already stated, Neurontin (gabepentin) or other anti convulsant/seizure medications are generally the first option. Neuropathy is difficult to treat however, so if the pain is neuropathic (burning or crushing sensation not helped by traditional NSAIDs or Opiates) then generally a low dose tricyclic antidepressant is prescribed. If none of the above work then direct lidocain injections or biofeedback are really the only other options for relief.

Low dose antidepressants are probably worth looking into anyway. Due to brain chemistry we actually feel more pain when in a depressed state, either situational, chronic or chemical imbalance. It's important to remember that opiates depress the central nervous system and lower testosterone, causing depression, you can see the cycle developing here and one of the reasons opiates lead to addiction.

My advice is to talk to your Medical Oncologist about an antidepressant and TRT, or Testosterone Replacement Therapy. Stress, Opiates, anti-depressants and other medications all constrain testosterone levels in the body, there is a direct link between low testosterone and depression. There is also a direct link between lean muscle mass percentage and survival rates in cancer.

As testosterone is a key hormone in building/maintaining lean muscle mass, having it constrained as much as it is during treatment is believed to be one of the factors in Cachexia, or cancer related wasting syndrome, where the body sheds massive amounts of weight, almost entirely lean muscle mass.

Hope this helps, sorry for the book.

Eric