I dont have much time since I didnt get to this until late, so I will suggest these recent threads might be helpful for you.
http://www.disboards.com/showthread.php?t=2766812
http://www.disboards.com/showthread.php?t=2766831
Also, visit post 2 of the disABILITIES FAQs thread, which is about renting
ECVs and wheelchairs. You can find that thread near the top of this board, or follow the link in my signature to get there.
For the convincing part, I have 2 suggestions.
One is to pick out a place that he knows well that is about 2 miles from your house and ask if he could walk there.
Hes likely to say no and look at you like you have 2 heads.
If he says he could, ask him if he could walk there and back. I can pretty much guarantee that he will say no to that, but if he doesnt, then ask if he could do that every day in a row for the number of days you plan to be in the parks.
There and back would be 4 miles.
The distance to walk from the park entrance at Epcot to the entrance area to WS is over a mile. The distance around WS lagoon just walking on the main paths is 1.0 to 1.1 miles, depending on how close you are to the water (staying very close to the water is a shorter circle).
If you go to see Soarin, your walk in the queue will be 1/4 mile and after you are done, another 1/4 mile to get back out to the ride entrance again. and that doesnt include walking the steep hill to get into the building in the first place.
Estimates of the distance people walk in a day at WDW range from 3 miles to 9 or even 12. The average is probably at least 6.
And, most of the resorts are large, so it is easy to walk 1/2 mile before even leaving your resort.
The second thing to think about is my FIL. He had bad knees and hips, but toughed it out. He thought about renting an
ECV every year for a number of years, but just didnt do it. We watched him in pain and not able to walk far enough into the park to do many of the things he wanted to do. He finally rented one at Epcot one year and said he wished he had done it sooner because he saw more in that one day than he had in his last 5 years of walk a little, sit a little, walk a little, sit a little and go home in pain by late afternoon. He also said it was the first time in years that he had not ended a day in pain.