He had the accident/contracted the disease when he was about eleven or twelve, and has been walking with a cane ever since.
It's very important that it DOESN'T cause him any long-term physical pain. The plot hangs on him not being on any sort of painkiller.
My original theory was that he was in a car accident, and shards of glass severed the tendons above his thigh, but I've recently learned that there's corrective surgery for this.
Other than the limp and need of a cane, he's fairly able-bodied and mentally sound, so something like MS or early ALS is out.
Does this sound like something you've heard of? Any ideas?