Zip tie his string and cable together before hunts?

Bad situation for sure, other than talking kill zone with him and letting him know it's not fair to the deer I don't know what you can do without cutting him off. It sounds like he knows he isn't a very good shot with all the excuses he is making after target misses. He is probably to proud/stubborn to ask for help in his shooting, anyway you can work a crash course in shooting with him before season?

Do yo really think he "practiced" all winter? Is he struggling pulling the bow, does he have a high let-off, how's his form? Just some questions that came to mind, I would immagine if he were an older gentleman and didn't practice keeping his muscles up that it would be a problem shooting at 67yrs old. I know your not his babysitter, but hopefully he isn't just blowing smoke about practicing.

I know bad hits happen to everyone if you hunt long enough (I've had my share), but I get sick when I think about them. After any bad hit that I've ever had I have always drove myself nuts checking equipment and flinging arrows at the target. I have friends that chalk it up as "no big deal" and get on with it. I guess it just doesn't bother some people as much as others.

Sorry I couldn't offer any useful solution to your problem, but it is a great thing your doing letting him enjoy the woods while he still can. I am sure it means a lot to him.