@dbk0025,
Dbk0025, this may not be the movie you are looking for, but it is the best I could do. In the 1949 movie “The Secret Garden” Dean Stockwell playing the role of Colin Craven talks about lumps on his back. It is not a comedy and Dean Stockwell is not portraying anyone who is mentally impaired, but since no one else has replied I thought I would give it a shot.
Good Luck in finding your movie.
Tom,