I feel very heartened by the sensible, considerate comments here. My mother is 90 and up until the last few years her life has consisted of going to meetings and out in field service. She is entirely disabled now which is awful for someone as dedicated and active as she has always been. When I hear that she has managed to physically get to a meeting (a very rare occurrence these days) I always praise her and tell her how thrilled I am for her. There are times when, out of compassion and kindness, we should keep our personal feelings to ourselves.
My advice would be to wish your dad well and praise him for his achievement. It will mean a lot to him, and in years to come when you look back on your memories, you will be so glad you did this