My son just called a girl on TV ugly. What should I do?
He was probably trying to get your attention -- and it worked! But why he chose that particular insult indicates that he's heard that language before, whether on TV or from friends, so it's worth exploring what's behind the comment.
Kids age 5–7 are becoming aware of what gender they are and figuring out their gender roles (that's why so many first-grade playgrounds are filled with games of girls-against-the-boys). It's a critical time for developing concepts about what girls and boys should act and look like -- and you can help him broaden his definition.
Rather than admonish him, use this as a teaching moment. Casually ask him: Why did you say that? Why do you think she's ugly? What does "ugly" mean? Is it bad for girls to be ugly? Listen without judgment -- and then drop it. Find opportunities later to share your opinions about appearance and gender. You can do this through explicit discussions or by modeling the type of reaction you'd like to see. Try pointing out behavior rather than appearance ("Wow, she worked hard to figure out that problem!").