What specific narrative style does updike use in "the slump"?

I would say that the style of narration in "The Slump" is specific because there are no dialogues - the entire story is told in the form of a monologue, inside the protagonist's head. We see that the narrator is telling the story as he is experiencing it in his own mind, and Updike probably used a technique such as 'stream of consciousness' to convey his rambling thoughts.