Congrats on the USMA appointment!!! My DS was in the exact same situation last year. I would have your DS send an email to the following (explaining that he has decided to accept the USMA appointment):
1) Cadet Command;
2) Personalized thank-you email to the AROTC ROO who interviewed your DS, thanking him or her for the opportunity; and
3) Personalized thank-you email to each AROTC PMS/ROO that your DS has met during his college exploration journey, so they can cross your son off their list and won't be expecting him in the fall.
Emails are easier for the ROOs to forward to whoever needs to receive the information, but a letter will work as well. Also, your son should nevertheless receive a letter from Cadet Command, asking him to confirm his intentions. Just send that in (or don't respond) when that happens.
Good luck!