Hi Marie!
I have some experience with this and my Father-in-Law. He was reluctant to go with us (me, husband, two kids and Mother-in-law) because he was afraid of leaving his business and frankly, he was worried about all the walking we would be doing. He worried that he wouldn't be able to keep up and would keep us from having fun because we would be pushing him along. He worried that nothing would interest him in Disney at his age.
My husband and I came to him and told him that we understood his worries, however, this was about memories. This trip is about the memories that the Grandchildren will have. It is also about being together and being a family. We asked him to look beyond his worries and at the big picture....life is short and we have to grasp at happiness and hold on. And happiness for a family is making memories...and making memories at Disney World is amazing and special. Those Grandkids will always remember having him there and sharing this time together.
We convinced him to go by using the Grandkids..and it worked! Ah, I can dish out a hot, steaming dish of guilt when I want to! But it WORKED! And he came with us and he loved it...My kids will never forget having him there going on rides and eating out. We rested when he needed it and we had a great time hanging out on benches even, because we were together as a family.
And life is short..My dear Father-in-Law passed away this past March. Suddenly. He won't be able to go back to Disney this Fall and we will miss his spirit so much. But I have the best pictures and memories of him holding my kids in front of Cinderella's Castle. Everyone is smiling and so happy. Which is what life is about.
And if this doesn't work on your Dad, tell him to write to me and I'll serve him my favorite dish of hot, fresh guilt!