My dad had (okay still has) the same issues. For my Dad he was worried once he started using a wheelchair, he'd never stand back up again. I think he hated the idea of people looking at him like he wasn't "capable". I vividly remember riding the peoplemover at
Disneyland three times in a row just so he could catch his breath from the pain. When the day was over he'd be in so much pain we he could talk for 45 minutes. I loved experiencing the park with him but it was stressful for me as a child. I didn't want us to have to walk any further than we had to so as to not cause him pain.
We're planning a trip for Jan 2011. I now have my own family and convinced he and my mom to join us. While my Dad now does mostly use a wheelchair (he has CP and his joints/muscles finally told him enough with the walking) he was worried about "slowing" down our party and "preventing us from doing everything"
We talked honestly about how I felt as a child, how I don't want my kids to see him in pain, how much I want him there to have my kids experience WDW with their awesome and fun Grandpa and how he's not much fun (or himself) when he's in that much pain. I explained how we walk 5-10 miles a day and how its tough for me (finished my first 1/2 marathon this mothers day). I gave him time to think about what I said and readdressed the issue a few days later. He never told me I was right but asked to see all the options (park rentals or from a different company).
I say have a good old heart to heart and hear why he is really concerned. You'll be able to understand the real issue better and then a better solution will present itself.
BTW when he does use a
scooter he "pretend" races other scooters for laughs from my son. He also use horn at times I wish he wouldn't, like when I bend over. Again my son

. He acts like he's putting on his own parade, waving to passerbyers. He got a family to follow him one time

... This January will be an adventure!
Good luck!