I have a long standing hereditary neuropathy, and while it started with most symptoms in the feet and legs it has progressed to my hands, with increasingly severe symptoms. It was not as sudden in onset as you describe, but over the course of the last few months it has begun waking me in the middle of the night with severe pain and burning and while I have woken up with numb hands and arms for years the burning and pain upon awakening are recent developments as is the general increase in hand pain.
Like your husband I am on long acting pain medication, 90 mg. MS Contin per day with oxycodone for breakthrough pain. Even with this the pain in my hands is distinctly worse than a few months ago.
It can be a normal progression of this disease for the pain to develop and worsen in the hands and arms.
Sometimes I think if we are on heavy pain meds the symptoms can escalate quietly and what may seem like a sudden onset was really normal progression masked by the pain meds. When the pain surpasses the threshold of our meds alarms go off.
Fentanyl is somewhat of a high end choice and seems to be reserved for when other opiates have failed to control the pain. I am not sure there is anyone on the forum currently taking it but I do not think that it differs significantly from other opiates in long term use.
You are fortunate that you have an understanding GP. Try not to worry too much about what happens when this doesn't work. There are combinations of meds that work and new things in the pipeline. If he has obtained relief in the past with pain management something will be there for him as long as you have a good doctor.
I am 53 and will be relying on medication for the rest of my life, so do a fair amount of research. Practical Pain Management, an online magazine for doctors, is an excellent source of information on the state of the art.