There is an opening date, and as stated right now he is probably being booted from the system because he is asking for 2017, and they are working with class of 16.
It is not AFTER summer when they open it, esp. since the 1st boards start in Aug/Sept. It is late spring, once they have cleared the 16 slate.
I know for AF it is @ end of March 1st of April they open, or at least they use to, but this yr it maybe later...who knows. The reason they open in the spring is because for many they will want to meet the 1st board which will require transcripts, recs, and PFA testing, which can only be administered by certain people, and candidates need to get these things in order prior to school closing for the summer. PFA is usually the biggie since if any of the coaches are there, they are there to train fall athletes.
One thing to think about from a proactive approach is you state he is 16, and most jrs. are at this point, however, if his b'day is in Jan/Feb/Mar/April during the selection process next yr he will be 18 and considered a legal adult.
This is why many posters will state they need to be in control of the process at all times, I am not inferring you are taking too much of a role, but at 18 ROTC as a part of the military will close the door on you in regards to certain aspects. They are usually very lenient and will talk to the folks, but if it something like a DoDMERB issue, due to patient privacy laws, even on your health insurance they will not break that rule.
Unfortunately, many times DoDMERB can be a lengthy process and the child may turn 18 during it, so if the candidate hasn't been the driving force in the process they can find it hard to navigate later on from the mid-stream perspective.