I read your question and thought of my own father, who used to write out a step-by-step itinerary for a planned trip and send a copy to everyone in the family - even people who weren't going on the trip and didn't know a thing about it!
I really think some people are wired this way. They don't just want their ducks in a row; they want them lined up so straight you can shine a laser beam across them and they measure up exactly!
You're going to have to hold on to your sense of humor with him. He's probably very worried that something will go wrong for lack of planning. Let him plan as much as possible, but let him know it might not be able to work out the way it's planned. If the means he has to make a plan b and a plan c, well, maybe he'll have fun doing that. Meanwhile, think of contingency plans yourself to suggest at the right time, so he won't feel stranded. Keep smiling!
The guy can't help it. Look for the funny parts of this and write them down because these are going to be great Grandpa stories for the next generation. :^)