Answer: I'm not sure I understand why you need to be respectful when he'd be one to say, "Horse biscuits." The first response that comes to mind is, "You'll keel by next week if you keep eating like that." Is he a holdover from the do-as-I-say-not-as-I-do era? Is he used to being king, and commanding attention accordingly? If not, I say respond to him in kind.
Question: Actually, no - he has always encouraged us to speak our minds. I guess the timidity is coming from me, not him.
Next time I'll say some variation of, "Or you could try quitting smoking and taking a walk around the block now and again. Oh, did I tell you about that cute thing the baby did?"
Answer: Sounds good, but skip the cute-baby-anecdote buffer. You sound like strangers, not father and child.
It's probably worth making the distinction here that I advise this kind of response only when he makes comments about his imminent demise. I don't think it's your place to initiate any nags about his habits, nor do I think it's productive.
Question: If I try, "Sounds like you're afraid that what happened to Grandpa will happen to you," then he reiterates that his solution to that problem is to die; he has actually been pretty explicit about that. Then what? I ask, "Can you think of any other way around it?" He says something about how it's too hard to quit drinking and smoking. Then I'm back to saying "horse biscuits" and changing the subject, because it makes me uncomfortable, and I'm not getting anywhere. Anything wrong with that?
Answer: Yes, actually.
Do what you can to get comfortable with the subject. Death is awful, but it's inevitable, and being able to talk about it, prepare for it, voice your fears of it, and generally just face it, will help you deal with it.
When he says, "It's too hard" to change his habits, then you can say, "OK, yes, it's hard. But obviously you find it hard to think about dying, too - as I do. I guess I'm just hoping you can be comfortable with whatever choice you make, whether it's to keep your same habits, knowing they might kill you prematurely, or to try to improve them, knowing there are no guarantees."
Be an adult about it, and maybe he will, too.
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