Congratulations to your son!
There are many reasons a cadet decides to leave a service academy. I certainly cannot list them all here, nor do I know all the reasons, I can, however, list some of the reasons based on feedback I have gotten from parents whose cadets have disenrolled.
1) Going to a service academy was not the cadet's idea
2) Went to play sports not realizing that they must be involved in much of the military training as well
3) The service academy wasn't what they expected (do the research!)
4) Does not like all the physical training, military training (knowledge tests), and/or being yelled at during their freshman year (C4C)
5) All cadets graduate with "Bachelor of Science", which means they take Calculus, Physics, Chemistry, etc their first 2 years. A cadet who doesn't like STEM may struggle during this period and decide to disenroll.
That's just a few things. My suggestion is to encourage your son to continue to train and run, run, and run some more. ... and relax! Enjoy his time with friends and family before I-day.