I can relate…
I can relate to what your father is going through right now. It sounds almost exactly like what I went through close to 22 years ago. I was given nothing for pain for fear it would interfere with my breathing. After 4 to 5 days of pure torture from the lack of sleep and pain that racked my entire body I finally went into respiratory arrest and was intubated. I was then finally given something that allowed me to sleep for a short period of time. I believe the drug was called Atavan. Back then PP and IVIG was not given and they informed me that all they could do was let the GB run its course. When I got off the ventilator after 2 months I remember receiving Motrin and Darvaset for pain which was very little help.
Thankfully there are better ways of treating GBS now and it appears that your father is receiving the benefit of them. I commend you and your family for staying with your father. This is the time that he really needs you all.