He didn't "do it", he was one voice among many astronomers who have been calling for a reclassification for years, the IAU voted and made the decision. It's a little silly calling him out for "doing it" for ego reasons when you are the one implicitly giving him credit for it... He didn't write the definition, he didn't chair the committee, he wasn't even on the committee. All he did was leave it off the list of planets at the Hayden Planetarium, where he was director.