Technically, the 31 January deadline is the "deadline." But, I advise my students to keep updating the SA admissions counselors on their details if something significant occurs, such as lettering in a sport or being promoted in a club. The worst thing that can happen is that 'they' ignore the update. There is no harm. My USNA candidates have a generic email address in their portal for the admissions counselor(s) for their region. I assume you son has the same. Be sure to attach some 'proof' of the news, such as an email or newspaper announcement, if you have it.
Lastly, I assume your son has a Plan B and Plan C. NROTC is a great alternative, and he can join a unit (free of charge) on campus 'next year.' I have had several students follow this route and then reapply to the Academy. All earned appointments the following year. Some 25-30% of all middies did not (and do not) come directly out of high school. The SAs are impressed when students reapply and show that they can accomplish college-level academics successfully.
Good luck with Plan A!