These categories are hardly arbitrary, they are political. We separate people into republicans and democrats, too, but just because this separation is not based on genetics or geography does not make it arbitrary or even bad.
Don't confuse genetic/geographic science with science as a whole. Anywhere you can do experiments you can do science, and politics is no exception (although everything is much noisier). Certainly, the groups of people described by the census are meaningful in a scientifically confirm-able way: they identify with each other, forming clearly discernible self-selecting cliques, neighborhoods, etc.
If he wants to say "the races as defined by the census are only weakly correlated with genetic distance or geographic ancestry", then fine. But that's a far cry from "the 'race' categories that are used in the United States are arbitrary...and do not correspond to any scientific view of geographical populations."