Jenny,
Tell your Dad he'll love it. We've brought my Dad with us a few times (he just turned 84 on 6/2), and he now gets bummed when we don't bring him with us! He has some mobility issues, but I rent a
scooter for him from Walker Medical; they have it waiting in Baggage Services when you get to the hotel & you leave it back at Baggage Services at check-out. The scooter is not a problem, the Friendship Boat captains are great, as are all the bus drivers.
My Dad last went with us in 11/2002 & will be going again this December. His favorite ride is Test Track, he loves the parades, and all the fireworks shows. Jammin Jungle at AK, along with the Lion King show were big hits with him.
Oh, one other thing. Your Dad being on the scooter will give you some new perspectives on WDW; some of the CM's at the handicap entrances are amazing. The Haunted House handicap entrance was an attraction unto itself......the CM had us in stitches, he stayed in ghoulish character the whole time!!
Go and have a great time. I know it'll be rough at first; my Dad & I lost my Mom to cancer too. Sometimes it just seems hard to have fun without her, but she wouldn't want it any other way. Good Luck!