Remind him that Harry was initially going to be assigned to Slytherin by the sorting hat, but that he ended up in Gryffindor because he felt that was where he belonged and influenced the hat. That's a major theme in HP--our innate traits don't define us; our choices do. People are often suited for many clubs and professions, and it is the combination of will and skill that determines fit.

What is to give light must endure burning.