To be honest with you, USNA really doesn't want to have someone in the Brigade who truly doesn't want to be there. A midshipman who determines that NAVY just isn't right can resign at any time. It will take a few days to out process, but that's about it.
Candidly, we would rather the appointee make this decision before I-day; it will be less traumatic, and we usually have some highly qualified candidates who are available to fill in the blanks. It isn't unique to have anxiety about any of the service academies, and your brother has a couple of months to work on his decision. He should talk this through with parents, counselor, any military he knows at least reasonably well, any midshipmen he might know to get the best picture he can. I am sure that there will be several persons on this site who can provide insights and experiences. Best wishes to him and anyone else who is in his quandry. Hurricane is an excellent example as a recent grad looking back upon her experiences and her prospects going forward. She is very direct, and that makes her counsel that much better.