I'll also welcome you to the PN forum - you'll find this is an extremely knowledgeable group of people who will share anything they can to help your father-in-law as he struggles w/ PN. As Dahlek said the causes of PN are almost myriad >100 different types and what helps some will have no benefit for others - everyones' physiology is different. You'll find very helpful people here who can guide your father-in-law about the benefits of vitamins and other supplements, nutrition, calcium, magnesium, attitude, exercise, Anodyne therapy, magnet therapies, deep infra-red heat therapy, etc, all of which can help reduce and reverse the symptoms of PN.

Re: your comment about the cold - for me that is the single most difficult type of pain I deal with constantly, extremely cold toes and feet for which no medication gives me any real relief. Sometimes my feet are in fact cold to the touch, but other times, like right now they are normal temperature but the dysesthesia registers as extreme cold in my brain nonetheless. HOWEVER, some months ago my wife bought a mattress pad that has a built in heating element - sort of like an electric blanket but underneath. In any case, I turn it on ~15-20min before I come to bed and the warmth on my feet and lower legs eventually allows me to go to sleep - thus far it is the only thing that usually helps me - maybe something similar might be of use to your father-in-law.

Best of luck in your search for answers - you'll find a lot of valuable insights here.
Alkymst