This is nuts because he doesn't realize that he's made an enemy of Martha's parents by acting like a complete geek when their daughter was showing off her boy-toy to Mommy and Daddy. What no Patterson ever quite manages to being able to begin to figure out is that Fred and Harriet would rather Martha date Jabba The Hutt because he has better manners and is less nakedly interest in her body. THIS takes the form of Mrs MacRae intercepting correspondence from "that piece of trash with the slicked up hair" so as to drive a wedge between an innocent girl and the worst member of a family of garbage people.
The reason that I mention this is that people are going to insist that Martha is so impressed by Mike's horrid prose style that she is cowed into silence. Since that would require Martha to be missing key chromosomes in real life, it makes a damned sight more sense for Mike to be lying about what happens to him next Summer. He writes his garbage to Martha, Harriet intercepts it, Martha has no idea she'd even been written to and we have an unproductive argument predicated on his inability to trust anyone unless they make him feel bad and his refusal to understand that as much as Elly hates his crush on Martha, her parents hate it more.