How much of a distance do you think your uncle can handle? Further, do you plan to visit all of the parks?
I think there are a couple of different considerations to take into account: Boat transportation to the MK is really the best option to drop him off closest to the entrance. But if you visit Epcot, you'd be using the monorail, which means making a switch at the TTC, and then being dropped off at the monorail station, which is about as long of a walk from the platform down to the entrance as it is from the bus drop off zone (maybe even longer when you consider switching monorails).
For Hollywood Studios, the closest drop off point is by boat.
For Epcot, the closest drop off point would be by boat in World Showcase.
For Animal Kingdom, it's by bus (for all resorts).
Therefore, I think if you're looking for the best way to get him to the majority of the parks (HS/Epcot), I'd look at an Epcot resort and use boats to get to both HS and Epcot, and use the bus for the remaining two parks. Both Boardwalk and Yacht Club are pick-up/drop off points for the boat, and both have launches a short distance from the lobby. YC might have a slight edge for me over BW only because the boat ramp AND the bus depot are the closest to the lobby. BW would also require going down steps or an elevator from the lobby to get outside to the boat ramp (which is also downhill and not nearly as wide or sturdy as the YC boat launch); also the distance from the BW lobby to their bus depot is further than YC.
If you're looking for the best way to get him to the MK, then I think I'd look at resorts that offer boat service to the MK, which would be WL, FW, Poly, GF and CR. The monorail pick up/drop off point is just about as far as the bus depot for the MK. And being on the monorail line doesn't offer a distinct less-walking advantage to get to Epcot over resorts that use bus service to the front entrance to the park.