I'll try to answer a few questions, but lots of research and due diligence on your (your son's part) will be needed. These forums are a great resource, so please read everything you can.
First he will need to visit an Army recruiter and find one that will help him. Do not let them convince him to enlist first. Street to seat is what you want.
As far as letters of recommendation go, choose the best 3 and submit those. Most board reviews only have a few minutes to go over your entire packet, and having 6 LoRs will just make it more likely the worst one will get read and the good ones will not even be looked at.
Percentage wise, civilian applicants only board against other civilians, so you will not be competing against people that are currently serving.
A competitive SIFT score is generally considered above a 50. But keep in mind that the ASVAB and SIFT matter less than the APFT, LoRs, and the essay.
For civilian applicants, younger people usually have it harder. It does happen and they do get selected, but you are generally competing against people in their mid 20's who have had jobs and been in proven leadership positions for a few years. Right outside high school is a little harder, but having JROTC could help there.
He likely won't have to interview since they just did away with that at the battalion level, so make sure the entire packet is strong.
Thanks for the reply XIIGage. We are in very beginning of this journey and we have given all responsibility going forward to our son. I will have him start posting any further questions.