Hi Laura!
Your father is very lucky to be included in your family vacation. Walt Disney World is a wonderful vacation destination for all ages and he will surely feel the magic as much as everyone else in the family.
I was able to take my mother to Disney until she was 90 years of age and it was a dream vacation for her each time. Unless your father is used to using an electric scooter I highly recommend instead using a wheelchair. You can
Rent Wheelchairs from the off site companies that you rent scooters from. I found navigating a wheelchair much easier and it was less stressful and more comfortable for my mother then a scooter would be. The parks get crowded and scooters can be very difficult to get around.
Consider elevated toilet seats for the resort room. They also can be rented from off site companies. Bring a good night light since the room will be unfamiliar to your dad if he needs to get up at night. Keep the days planned so there is a break time built into the itinerary. Make table service dining reservations where everyone can stop and rest and relax.
Make sure he wears clothes that are appropriate for the weather and that needs to include hat and sunglasses. Sunscreen of course is a must and keep everyone in the group hydrated. Even if the temperatures are not extremely high the humidity is what takes the energy out of you.
If there are attractions that he cannot or does not wish to ride then find shaded spots where he can enjoy watching the surroundings while he waits for you. Keep the family involved in his care and needs. When we had mom in the wheelchair we would all take turns "driving" her and had fun seeing who could make her laugh the most.
I am sure your family will bring back incredible memories form a vacation that can be shared by several generations!