On his Wikipedia page, in the 1966 - 1975 section it says it took a lot of persuasion to get him to use the wheelchair. This suggests that he wasn't in dire need of it at that point. His first child was in 1967, the last in 1979.
6. Extremely high ammonia levels in the water can damage the outer layer of skin and cause shedding. RES are tolerant of high ammonia levels while map turtles are not, so this problem is more likely to occur with Graptemys species.
Tortoises do not shed skin as dramatically as snakes or some lizards, that take their whole pelt off like a coat, but they do shed their skin every few months. Because their shell is in the way, they cannot shed their skin in one go, so it peels off bit-by-bit, in what can easily be mistaken for a skin disease.