Ok here is another Idea. Assuming Grandpa is in good condition, and you will need to get him to agree first.
Have him keep the bike for you. then go get your CB500X and ride it as much as you can in the next couple of months.
Fix it up and make it nice. Then when you are ready, trade grandpa your CB500X for his ST1300....
Now he will have a bike that he could ride and you move up. It could be a win win for you both and you get a riding partner.....
I bet his eyes get big, and he smiles..........
haha that’s funny. No honestly he seems to not have interest anymore. He is over 80...he is in pretty good shape for now but grandma isnot allowing him to continue riding and he agreed with her. It’s the riding itself not the size of the bike that she is concerned about.