Well in this story, Mr. Norton is one of the major trustees (financial contributors) to the black college that our protagonist attends (I will refer to him as the narrator for now on). During the car ride that the narrator takes him on, Mr. Norton blabs on about how fortunate the narrator is for having this wonderful opportunity to better himself, made possible, of course, in large part by himself (Mr. Norton).
In a very patronizing manner, Mr. Norton lectures the narrator on how "your destiny is my own." In other words, Mr. Norton is essentially telling the narrator that any success he might enjoy in life would be because of him (Mr. Norton). But Mr. Norton doesn't really seem to care about the narrator personally. This is borne out later when Mr. Norton fails to stand up for the narrator when he is booted out of college by Bledsoe and fails to answer the narrator's letter when he come to New York. At the end of the book, the narrator makes mention of meeting Mr. Norton in the subway and Mr. Norton either fails to recognize "his destiny" or has long forgotten about him.
In my opinion, Mr. Norton is characteristic of most liberals today. Liberals always make a big show of how much they help the blacks, but in reality, they are keeping them at a distance. They use social welfare programs to keep the blacks in their debt, hoping that it will pay off at election time.
You ask me why Mr. Norton was so fascinated by that story of incest by Trueblood. Well, during the ride with our narrator, Mr. Norton spoke in a curious way about his daughter, who apparently died at a young age while on a European trip that he and his daughter were on alone. Mr. Norton even carried a picture on him of her (that he showed to the narrator) that showed her in a dress of "soft, flimsy" material. More like the picture that a boyfriend or husband would carry around than a father. Perhaps that might explain why Mr. Norton was so interested in the story of incest.