That sucks man.  Watching grandparents decline is a painful thing.  My Papa is 84, and is slowly going senile.  Every time I talk to him, he asks me if I made the same rank yet.  Just remember the times when he was stronger.  As a society, we care too much about last words, which is a shame, because what matters is the things he said and did that made you this close.  It's hard as hell, but eventually the time comes where you need to be strong for him.  You'll get there.  It helps to talk to others that love him, and to dwell on stories of him in better days.