This is a very unfortunate situation for you father. From what you have written it does not sound although your father has been treated incorrectly but has simply suffered some recognized complications of this type of surgery.
There are some treatment options for your father from medication to steroid
injections in the spine and around the nerve
You should explore these treatment options with your treating surgeon who may also opt to involve some pain specialists in the treatment of you father.
I hope you find this information beneficial.