In my son's case, he finished the interview process way before his file was complete.
There was a major computer glitch at my son's HS regarding GPA calculation and it did not get fixed till last week of Sept, when the "correct" transcript was finally mailed. He did not do his physical fitness test until a couple of weeks ago.
Yet, he had an interview during the third of September.
I think once the online submission is complete, you can do an interview and supply all other documents later as long as the deadline for the board is met. Furthermore, my son was able to "tinker" with the online application till the day before the deadline for the upcoming board (tinkering as in fine tuning the school of intent list).
To initiate an interview, simply have your son call a PMS of a nearby battalion or any PMS he wishes to interview with. If your son has a "dream school" in mind, and if that school is nearby, I recommend that he interview with the PMS of the battalion of the school.