I don't think it's an impolite question to ask and I appreciate people who are straightforward. I would explain it just as you have here. Say "hey doc, I often worry about the future and one thing I really worry about is you retiring because we have such a great relationship and you also accept Medicare which is rare" he will understand your concern. He might feel immortal and say "I'm never going to retire, I'm aging backwards" in which case I wouldn't touch it and would ask him for his secrets.