Congratulations on the nominations. I'm not sure what you mean by interviewing with USAFA, but if he had a MOC interview for that particular SA, he may have NOMS for all 4 and that's great. It's one step closer to an appointment.
With that said, not everyone who is turned down for an appointment is guaranteed nor even in the running for a prep spot. The prep school is usually offered to those the SA feels is not quite ready for the SA. They have what the academy is looking for but perhaps they have been out of school for a while, or their test scores and grades need a bit of improvement. There are too many scenarios to explain here. But it is not automatic if they are not appointed and the number of nominations one has, has little to do with a prep school offer.