I don't want my boys to grow up to be the white-privileged folks whose words distressed me so very much this morning. I want them to become the fans of a better tomorrow (or some such sincere whatever). So I talk to them, and with them (and yeah, they sometimes get pretty sick of it, because they're kids). We talk about who's shown on covers, and how they are shown. Anybody non-white? If so, are they behind a white person, or obscured in some way? If there's a girl, is she behind the boy? Is she looking sidewise instead of straight on? Who has what weapon?
We talk about how people are identified in the books. Who's skin color is mentioned, and who's isn't? What are the adjectives used? Are there characters whose ethnicity you can't guess at? Ask them if they are imagining people who don't look like them in their own writing. This actually has had results--my younger son was very proud of himself for deliberately choosing to write a non-white character into the standardized testing writing sample he had to produce. And sure, it was tokenism, but at least he's recognizing that fantasy worlds don't have to be all-white, so I patted him on the head and told him he was a good child.
And finally, I offer them books whose protagonists aren't white boys. It's easy to find great books with girls to offer boys, but it's harder to find great sci fi/fantasy for kids with non-white protagonists. Here is a quick list of some of my favorites, that I promise most white boy readers will love:
Where the Mountain Meets the Moonby Grace Lin (2009). A beautifully illustrated Newbery Honor winner set in China.
Bansi O'Hara and the Bloodline Prophecy, by John Dougherty (2008) An Irish fantasy starring an Indian/Irish girl.
Chronicles of the Red King: The Secret Kingdom (2011) by Jenny Nimmo. Magical fantasy whose hero is African.
Geeks, Girls, and Secret Identities, by Mike Jung (2012). Fun superhero adventure with an Asian boy.
The Menagerie, by Tui T. Sutherland and Kari Sutherland (2013). Readers might not pick up on the fact that the main character is an African American boy, because it is a non-issue, but you can point it out them.
I have many more in my list of diverse fantasy/sci fi for kids, but these are the ones that I feel most comfortable recommending.