How long does it take for a nerve that has been severed in the foot during surgery take to completely heal?

My dad had surgery about 2 years ago now the Dr. that did the surgery severed a nerve in his foot. Since then he has been from doc to doc to see if anyone could possibly help relieve the pain with no success. He describes the pain as it you were to wrap your foot in barbwire and stick in a bee hive. He was told that nerves heal from your head down and since his is in his foot it would be a long time for it to be completely healed. There is not a day that goes by that he is not in pain and has been so since the surgery. He has been on a number of medications for the pain and he says all they really do is make it so he can tolerate it a little better.He has also tried the TENS Unit with no change. We just want to no if he will have to live with the pain in his foot for the rest of his life? How long does it usually take for nerves to heal? And what can be expected in the future? He is up for any suggestions that may help make it more bearable.