Some strong and solid advice from all who have posted here.
First off, thank him for his service from me. Secondly, talk to him. If you want to know something, you just have to ask. If you want to tell him something, he will not know unless you tell him. Seems pretty simple from the outside, but I know it cannot be easy for you.
Sounds like he is a true American hero and a great father to you. Sounds like a fighter and this will serve him well as he battles his illness. Can't say I have been in your exact shoes, but my dad gave us a big scare when he was 59. It showed me my dad was not invincible. It was not an easy road, but he is still here and I know to enjoy our time together and never take it for granted anymore. You dad may be going down hill, but he isn't dead. Sorry if this sounds blunt, but sometimes there is no other way to put it. Enjoy every minute with him. Whether he is around a year or 40 more years. Talk, laugh, go to lunch, or help him clean the gutters. Whatever you do with him or for him. . . .soak it up.