No, I think he's right. Right now planets can be said to be defined by roster. That is, a planet is whatever somebody decides to call a planet.
Most planets have orbits near the ecliptic plane, with relatively low ecentricity. Pluto fails these tests, its orbit is inclined with respect to the ecliptic plane and it has a relatively eccentric (oval vs. round) shape.
Pluto's discovery was an anomaly. Many astronomers have wanted to remove it from the Canon for years. I think there will be resistance to adding more planets. I suspect anything found orbiting the Sun from now on will be classified as a Kuiper Belt object (or similar definition), Pluto will retain his status as a planet as a matter of tradition (since 1929!) but with the qualification that he's the only planet who is also (read: "actually") a Kuiper Belt object.
Ceres lost its designation as a planet once many more asteroids were discovered, and Pluto will as well. It's only a matter of time. Pluto is quite obviously a Kuiper Belt object, with the only distinction that it's usually the closest sizable one (Quaoar gets closer for part of its revolution).
PS. It's estimated that there are between 20 to 30 undiscovered planetoids the size of Pluto lurking out beyond Neptune.