Could be lotsa things. I will echo the sentiments of a few other's here and say that it might have just been too hard, or he plain didn't like it. I graduated from West Point in 1998, and between my first day and graduation saw lots of people leave for a lot of different reasons - one thing they all had in common was they didn't usually like to talk about it.
Most of the kids that go there have breezed through high school, scored really high on tests, been student leaders, athletes....in short, overachievers who have never failed at anything.
Once these fair haired children (not unlike me 5 years ago) arrive at the military academy, they are immediately put in an adversarial situation with 3000 upperclassmen, given a ton of really difficult homework, military duties, mandatory athletics, physical fitness tests...which gives our hero's a choice: kill themselves and try to do everything perfect, do some things perfect and blow - off other stuff, or half-ass everything and accept a less than perfect grade every now and then. This is a bitter pill to swallow for an 18 year old who has always been the smartest, fastest, most favorite of everyone he's met up till then.
I doubt very much that students are leaving the AFA in droves - I heard some rumblings about a drug scandal, but when things like that come up they are usually handled pretty swiftly (and harshly) and then quickly forgotten when some cadet runs across the central area naked throwing MRE bombs at people during finals week.
Anyway, in summary after much e-rambling: lots of kids leave the first year, most of them feel like they have let their families down by deciding that the military wasn't for them...imagine how you would feel when your first real failure in life is dropping out of a "prestigous" university. I would encourage him to pick himself up and move on. If he didn't like the academy, he damn sure won't like the military - I'm not too crazy about it right now either, to be honest, so he may be saving himself 5-6 years of post-graudate heartache.
Hope these ramblings of a cynical Cavalryman help some.
Beat Navy, Load Sabot---
Tanker