Light Yagami/Harry Potter
Vernon and Petunia were the type who thought their darling son was a genius. They would absolutely refuse to believe any diagnosis of Dudley's impediment. Would refuse to go to a doctor for it. And thus, it wouldn't end up in Dudley's file and his teachers wouldn't know to account for it. Dudley would end up absolutely hating schoolwork and class and that would effect his education and scores.
Thus, Light's horror. He would have to be the responsible one. For Dudley's education. Him.
Dudley better appreciate it. Light hadn't spent half this effort behind Sayu.
So, at six years old, Light took Dudley at his side and spent their evenings reading to him aloud from books, covering up letters as much as he can with his tiny hands and making sure Dudley got each and every word and letter correctly.
It helped that he had already taught the child to speak clearly beforehand. Cruel, but something Light considered important in the light of this new revelation.