I remember waiting for the call about my grandmother. Her death was a much better thing than the end of her life. Alzheimer's had taken her mind years before, and there was no dignity left in her existence.
It's wonderful that your grandfather can decide his fate lucidly, and it would be even more wonderful for the people around him to let him know they accept it.
I wish that things may go as well as they can, and that you can celebrate no matter what may come.