https://www.thebalance.com/can-a-non-u-s-citizen-join-the-united-states-military-3354092
I had to sort through about 120 ROTC cadets in my Region back in the mid 2000s who had clamied dual citizenship. Each one had to send a registered letter to the embassy of the other nation declaring they denied that nations citizenship and were solely US. Those who refused, scholarship or not, went to the door. Scholarship Juniors or seniors found it a costly decision - recoupment. Seems that for a few years college PMS’ were not fully confirming such status and the result was a mess.
AFA your childs Doc, unless you know his citizenship as a fact, I dunno.
Officers are agents of the US government, enlisted folks are not- hence the appellation “officer”_- holding an office of the US. Officers are appointed( commissioned) by the service secretaries or by the president rank dependent and approved by congress- just like other federal office holders.
As a retired Army officer, I will hold that rank and title until I die or by law am relieved of it.