He could find out anytime between next week and mid April. Prior enlisted are sort of handled differently as there are other factors such as age that come into play. Many times they sort of wait and see who they all get and rack and stack them for how long they have been out of school, academic resume, leadership accolades and potential, maturity, age, etc.
Reasons for prep school are for academic tune up. It can depend on their academic credentials and how long they have been out of a classroom for those selected for prep. It can be tough to be out of a traditional classroom for 3 years and be sent to USNA. NAPS gives a great prep for to adjust to academics and the expectations of being in an officer pipeline. They look at a lot of factors to determine if prep is a good option or not. To be honest, it’s a great option. If offered, take it! I know one young man who was at the age cut off but had been taking college night courses. He was a direct admit because admissions saw he could handle Calc and Physics. It really varies and they want to set the appointee up for success.