Joe Pepitone is important in Okay For Now because he represents all that is right in the world for Doug.
There is not much right in Doug's world. He lives in a world of physical and emotional abuse. There is very little good in the world that Doug can authentically call his own. However, in the opening of the book, there is a beaming of pride in how "Joe Pepitone gave me his baseball cap." Doug idolizes what the cap and jacket come to represent. He feels that being able to being able to play catch and bat with Pepitone was a moment where everything good converged.
At the same time, Joe Pepitone represents something that is just for Doug and not having to be shared with others. Doug says as much with, "It was the only thing I ever owned that hadn’t belonged to some other Swieteck before me." The sadness and resignation he feels when his brother abuses him and trades it is reflective of the pain in his life. If the life that he leads is a desert, then Joe Pepitone represents the small instant where a garden emerges.
Joe Pepitone is a hero for Doug. When Doug must act in front of his idol and then when Pepitone comes back stage, it is a moment that Doug can never forget. This experience underscores the importance of Joe Pepitone embodying something good within a world of bad for Doug. It is the faith in people like Pepitone that enables Doug to believe that he is "okay for now."