I would just turn the comment around on him, saying something like, "I have grey hair because I'm old enough to have learned a lot. And if you would get out in the sun more, you'd be able to have nice dark skin like mine, too!" That shows that you're comfortable with your differences, helping him understand that differences do not imply superiority or inferiority.
If you think he was purposefully being rude, you could also let his parents know about the incident.