Dan it's great that your dad is able to be aware enough to make the final decisions his own. You are so lucky to have him for that long, and to be able to gain as much wisdom and lessons from him during that time.
Remind him of that. Remind him of all the times that you've learned from him, how you can pass those lessons on to those who follow. That you will share his stories, his experiences, his legacy. Remind him that things are going to be okay because of what he gave you. That he gave you the best that anyone could have given to their son. And thank him. Thank him for life. For making you who you are today.