Actually, AROTC does not take senior year academics into account for the scholarship application. What he can put on his application are any clubs, leadership, captain of athletic teams and varsity sports for his senior year. It is sometimes hard for the kids to anticipate what they will be doing their senior year so it's ok to wait until school starts to make sure he gets everything included.
Like it was said above, the first board is October 21st so just make sure he has everything in at least 2 weeks before, he can submit what he has and then make adjustments as well. The essay is important but not as much as with NROTC since it's only one. Just try and stay away from too many clichés and speak to why he wants to do ROTC and serve in the Army. If he can't get in touch with his english teacher, have others read it, see if there is an English Professor at a local college that can review the essay, not sure a recruiter will be the best place, he could end up walking out enlisted in the reserves. Most recruiters won't have the background your looking for in an essay review.
The scholarship process is actually a rolling admissions in a sense. There are 3 boards, if he is not selected on the first board his application will be rolled over to the next board. Applicants can wait and submit their applications for the second or third boards if the choose, it is better to get it in for the first board so he has three chances.
On thing to remember, once the application has been revirewed by the board he can't make any changes or additions other then SAT/ACT scores or School choices, so make sure he has everything he wants on the application when he sends it in.
If he is not sure but anticipates being in a leadership position or a varsity sport after the application deadline, it's best to put it on the application.
Best of luck to your son.