Although I did not have the same surgery as your father, I underwent similar treatment. I went to physical therapy after I completed radiation to help with the movement, which I believe a lot of people on this site have done. My neck still gets sore/tight sometimes, but I am constantly stretching and rolling my neck which also helps. It seems the movement came back with time... right after surgery, my neck was very stiff and weak, but by the time I started therapy most of the movement had come back. The occupational therapist/physical therapist should be able to give him some exercises to do as well. Good luck!