The grumpy old man helps Maniac see the prejudice all around him in the East End. Gee, thanks, grumpy old man.
Yeah, this old dude is mean, but something had to give. Maniac had no idea that he was causing serious problems until this old guy spells out the reality of race relations in Two Mills in front of the fire hydrant: "'What happens when we go over there? Black is black! White is white! The sheep lie not with the lion! The sheep knows his own! His own kind!'" (17.20).
Of course, by the end of the novel, we find out that the old man is wrong. But Maniac has a lot to learn before then.
This good Samaritan offers Maniac his first clue that there's more to the East End than meets the eye. Maniac and Mars are in the middle of their first big brouhaha, when she steps in: "This time Mars Bar was up against an East End lady in her prime, and she was matching him eyeball for eyeball … And when it was over, only one glare was left standing, and it wasn't Mars Bar's" (10.55).
Right after this, though, she lets Maniac know that he is treading in dangerous territory: "A little of the glare lingered in her eyes. 'You better get on, boy, where you belong. I can't be following you around. I got things to do.'" (10.57).
These East End folks are like guides: they help realize that not everyone thinks or sees the same way he does. And that's a major step in growing up.