Since your son is prescribed these psychiatric medications, I'm assuming he is already in treatment. Be sure to convey the information to his mental health providers.
We've haven't approached him with this letter yet. Is there anything we should say - I feel that he needs to have some kind of punishment.
Tell him you saw the note and are concerned/worried about the things he said to this girl. Let him know that you are informing his providers because this is something he needs to talk with them about. As discipline, remove some favored privilege (e.g., TV, video games, computer) for several days. And be sure to have him write a letter of apology to the girl.