Is there any hope of getting him to understand that this quiz is stupid? It was not written in letters of fire across the face of the universe, by the hand of God or Dumbledore or J.K. Rowling. It is just a thing made up by some person, probably with not very much deep thought put into it. It means nothing about your DS as a person, nor about what house he would end up in if the Harry Potter world were real.

If he's ready to hear that message, it could be a real teachable moment about how some things that seem to be authoritative really aren't at all.