I think you could let your son decide whether he wants to participate or not. Maybe he could try for some time and see how he likes it. Make sure that he understands that he doesn’t need to do that and can back up at any point. You never know if someone is interested in something until they try it, especially at this age. He might be interested in all these things he’s never seen before and probably never will. I get that it’s hard for him to say ‘no’ to people, so it’s important to explain that if he doesn’t like to do this project nothing bad will happen. He won’t disappoint anyone and can still spend time with his grandpa outside the project.
I wouldn’t give a confident ‘yes’ to your father either, he must understand that not every 7 y.o. could possibly be interested in film-making especially old ones and without watching them. So maybe he could include small film sessions with popcorn and stuff to make it more interesting for the kid. If your father is determent to involve your son in this project, I bet he is willing to adjust any of his plans of how to do that. Because if both parties aren’t having fun, then what’s the point of it? If this doesn’t work out now, he can wait another year or two to try again. Or start it on his own, and maybe later your son joins in seeing that his grandpa does something so interesting and attention-keeping. Kids are curious and they love doing “adult” things so your son might actually get invested in learning about films from his grandpa.
Let your son see what the reels are and if your father likes to explain things then there’s a big chance that your son’s attention will get caught and he will enjoy this activity. I wouldn’t shut this opportunity up without giving it a chance. Good luck!