The "name" of "Freak the Mighty" is not really the thing that gives Max more self-confidence. “Freak the Mighty” is simply a label for the powerful friendship and incredible team that exists between Max and Kevin. It is that friendship that empowers Max.
Max has the problem of a lack of self-confidence from the beginning of the book. Even from a very young age, Max is a child acting out because he does not feel smart enough to be among his peers. Max retreats “down under” into Grim’s and Gram’s basement in order to escape the world. Max initiates the friendship between the two boys by offering to get a toy out of a tree for Kevin. The two soon team up in order to have adventures and beat the “bad guys” who end up being Blade and his gang as well as Max’s own father, Kenny “Killer” Kane.
The literal name of “Freak the Mighty” is the description of Kevin sitting on Max’s shoulders. Max is the “mighty” one here. Kevin has always been nicknamed “Freak” because of his birth defect. The name means nothing next to the power of the two boys together. In reality, they could have simply been called “Max and Kevin” or something else. The bond between the two boys would have been the same and would still have boosted Max’s self-confidence.
Max’s new self-confidence due to the friendship continues even after Kevin dies. At this point, Max can no longer be “Freak the Mighty” with Kevin. However, Max can immortalize his friend by writing down their adventures.