"Vous êtes un garçon américain" is correct, because "vous" is in the polite singular form.
I can't be the only one who started singing "American Boy" by Estelle?
I though children were always in the singular...tu.
I thought children were always addressed as tu
Is it common to refer to a youth (singular) using the "vous" form?
Yes, in particular if you don't know and that you feel that he expects to be treated as an adult and not as a child.
I also made the mistake of assuming that the Tu form was the only correct answer, but apparently some figures of authority such as police may use vous to talk to singular children as a sign of formality.
What is the rule in French for capitalizing proper nouns ?
We capitalize proper nouns, not adjectives: je suis français - je suis un Français.
Thanks. In English it is more complicated.
"T'es" un garcon is marked as wrong
It is not proper French. In writing, you cannot elide "tu".
What the role for the noun proceeding the adj french
85% of French adjectives are placed after the noun.