One might understand the words of the mother and son in ironic relation to the title of the story, which is also the tile of the collection of short stories in which it was published.  Following this direction of interpretation, the mother “rises” to childhood innocence as she descends into death; however, the son, though speaking in the language of a child’s love for his mother, does not rise, but rather remains in a quagmire below his mother, knowing he loved her but will not be able to demonstrate that to her. One critic puts it this way:  “What is left after [the mother’s] stroke is certainly a very immature soul, but one relieved of accumulated error. Meanwhile, her son is forced into the world of guilt and sorrow where he might [only might] outgrow his selfishness and accept responsibility for his destiny. This possibility for growth is all that O’Connor usually allows to her sadly human protagonists. She does not concern herself with saints.”  Following this reason, mother and son do not converge at the moment of the conclusion but remain as sadly apart as they have been throughout the story.  Convergence, which is harmony and unity, is only a possibility, not a fact.

At the end, Julian's mother is having a stroke which causes her to become disoriented about where she is, etc. She's dying, so it's natural that she would think of her happiest days on the plantation before she dies. Julian wishes evil on his mother just before she suffers her stroke, so when it happens, he refers to her as "darling," "sweetheart," and "Mamma" because he realizes what his cruelty toward her has done to her. Julian isn't saying these words because he's gone back to childhood; he's expressing them because he realizes he loves his mother, and these words show that love for the first time.