I would say teaching Grayson how to read was one of his best accomplishments, one he would be proud of. The old man had few friends, and he gave Maniac not only a home, but companionship as well. Since Maniac listens to his stories, he finds out Grayson's story of a near miss at fame and lost opportunities. As the only one at Grayson's graveside service, Maniac gave him the honor that was due.
Another proud accomplishment might be keeping the young McNabbs in school as much as he did. Maniac was the only one who actually cared for and about the boys. Hopefully, he broke the cycle of bigotry and ignorance by influencing the two littlest McNabbs.