I'm actually conflicted over him being called 'UnAmerican'.
He's actually a greater American for refusing to stand. While it was, at best, semi dis-respectful to refuse to stand, at the same time, he did exercise his right as an American.
Isn't the whole idea of America based on the right to choose? Look how often the Constitution refers to freedom of expression and what have you.
Having said that, don't cry, it's only teenage wasteland for this guy.