Supposedly Charles had an identical twin brother who was brain-dead, and despite that he was still kept alive and on health-support for like 60 years...Also, this twin happened to have enough body deterioration to only affect his legs, not the rest of his body...and by the way none of this is even hinted at all in the movie.
Honestly, a better explanation is that Charles just takes over the movie universe's Patrick Stewart. Patrick Stewart got into a severe car accident and was in the hospital. When Charles disintegrated he transferred his consciousness over to Patrick's body. Charles thus ended Patrick's movie career, for the sake of obtaining a body that looks like him.
Makes more sense than the twin brother theory lol