Your youngster will probably happily email you with the details/or may have already told you what his mailbox number is. If I remember correctly:
Candidate John Smith - 2015
United States Merchant Marine Academy (or USMMA if you prefer)
Box 123
300 Steamboat Road
Great Neck, NY 11024-1634
After Indoc, Plebe Candidate John Smith - 2015
After Acceptance, Plebe John Smith - 2015
After Recognition, Midshipman John Smith - 2015
...
Heck, the mailroom guys were able to get stuff to my mid no matter how improperly addressed I scrawled things... I should mail THEM something (that would be delicious and definitely NOT on NCAA approved snack list). Those guys did a fine job of keeping us connected to our mid.