If we have a correct understanding of fallen human nature and OS, we can conclude that all human weaknessess, whether they are physical, mental, emotional or spiritual disabilities, have resulted from this.
While the parents themselves could not be blamed as individuals for the disability of the son, our collective fall from grace through OS certainly can. Man, as originally created by God, was not meant to suffer as we do now. Yet since we do indeed suffer so visibly, God in His great mercy and love, can bring forth greatness and beauty from even the most desperate human conditions.