The disease that Kevin has in the book "Freak the Mighty" is Morquio syndrome. The author, Rodman Philbrick, identifies Morquio syndrome as a rare form of dwarfism.
In the book, Kevin is described as using leg braces and crutches. This seems to correspond with the abnormal bone development and knock knees that are typical of patients with Morquio syndrome.
Morquio syndrome causes bone problems that can lead to paralysis and potentially fatal cardiac complications. In "Freak the Mighty," Kevin suffers from a seizure and goes into the hospital; when the narrator, Max, returns to visit him in the hospital, he is told that Kevin died because "his heart just got too big for his body."