Okay. I am applying from Great Lakes so we have two different groups of Admissions officers that work specifically with us. If you were from my region I would have PMed you with some pointers but I don't know how your region works as opposed to mine.
I'll give you all the general information I can, as I too had to go through the waiver process. It isn't exactly what you'd call exhilarating and can, in fact, be a daunting challenge. Don't worry, though, because I have seen a lot of friends obtain waivers for previous tears and fractures. Most of these friends were told by DoDMERB to schedule an appointment with a doctor to 1.) acquire an updated x-ray of the formerly damaged area, 2.) ensure that the formerly unstable area is now stable and capable of enduring strenuous military activities, or 3.) do both of the above.
Now, I don't know what they'll have your DS do, but his first priority is to ensure that a waiver is requested on his behalf in the first place.
This will depend on a variety of factors, some of which I am unable to specify due to the simple fact that I am not an Admissions officer and do not have a vast amount of experience with them. According to what I've been told, your DS's competitiveness is mainly what will influence his possibility of a waiver request. If his RC deems him to be competitive enough (and I have no idea what these standards specifically are), he will grant him the waiver request. Keep in mind that his RC can put his request on "hold" and refrain from submitting the request for a waiver until something about his candidate file changes.
I recommend that your DS email your RC asking about the waiver process. I understand that he doesn't want to bother him, but RCs exist for the purpose of assisting applicants through the fantastically long adventure that is the USMA applications process. Your RC will not be able to determine your son's probability of receiving a waiver, nor will anyone on this site. We are not doctors. Only the USMA surgeon can give him the clear.
I hope this helped, and go Army!