Scott,
So sorry for your loss. As many have said, remember all the good times. I turned 59 today, my dad is 82. Starting in Sept. through winter, I usually spend my Saturdays cutting wood, dad will call on Fri. nite and say "what time we cutting wood tomorrow". I will get up early and cut a couple loads then go pick him up once it warms up or thaws out to help eliminate some of the mishaps he could have(falling). I hate to not have him come with me, but on the other hand, as you mentioned, you have to watch constantly. Don't beat yourself up, he went doing what he loved with a great son.