I am guessing that admissions has decided to seek a waiver?
If your son is granted a medical waiver then he is eligible for an appointment. He is competitive but only admissions can tell you where he stands.
If he is the top candidate in your congressman's district he will get that spot, as long as he gets the waiver. If he isn't the top candidate then he will still compete in the national pool of candidates.
Have your son give his Regional commander a call - he can tell him where he stands.