As with many other posters, I now wear aids but resisted and was in denial for many years, 'me, deaf? No, it's you, stop mumbling' etc.
No aid, be it knee, hip, hearing aid or glasses is as good as the original - in good working order but wearing hearing aids will change you and your father's lives immensely, for the better.
The longer your father prevaricates, the more difficult it will be for his brain to be trained to accept them plus, there is the issue of possible faster dementia and not least, a family row if you are continually having to shout at him!
I really did think my family had conspired to mumble as a group!
Are you able to print out these responses and leave them in a strategic spot for him to find?
Wishing you both good luck!