When Dill becomes upset about Mr Gilmer's cross examination of Tom, why does Mr. Raymond think that he won't react like that in a few years?
He tells him this because he thinks Dill will catch what Atticus calls "Maycomb's usual disease," which is racism. When a person is young, Mr. Raymond theorizes, they tend to recognize injustices more clearly. However, once a young person begins to mature, they get used to the racist views of the dominant culture in Maycomb. Then they stop seeing anything wrong with it, and soon they don't notice it anymore.