[sosia]

The usual way, as explained above, its "eres". "To be polite" it's "ser educado"
-Eres muy cortés
-Juan es un joven muy cortés.
-Puedes ser cortés cuando quieres.

You can use "estar", but only when it means a momentary status, like "cuando quieres, estás muy cortés"
Here is an irony inside, you're actually saying he is usually not polite, but he is being polite now due to interest.
You can say "cuando quieres, eres muy cortés", wich is the usual ironic way, but saying it "estar" you say the same but you're more wicked as usual.

About the sentences:
-puedes ser cortés cuando quieres
-puedes estar cortés cuando quieres , but only in wicked times
I´m with your spanish expert

Saludos