Yes, as others have said, the letter is normal. I and others have TRIED to prepare some of the applicants here with HOW TO WRITE A LETTER HOME. But of course they forget a lot once they get there. 4.0 gpa students become ********. Your day becomes a blur. Time is measured one meal to the next. There is NOTHING that can totally prepare you for the academy. That's why there was some cynical remarks about the applicant who wants to be the first man to Mars. Boy Scouts, Civil Air Patrol, etc... will NOT prepare you. Matter of fact, you're probably better off psychologically if you didn't go in with any pre-conceived opinions.
Having said all that; you basic cadet will change moods hourly. Not even daily; but hourly. Unfortunately, you are the ONLY thing they have that they can call NORMAL. And unfortunately, they are venting to you. The best advice I can give to you is this:
LISTEN to what your child writes, but DO NOT put a lot of weight into the actual words. Look past the feelings you THINK you read into the letter. Give the following advice to your son/daughter. Tell them that there are 1347+/- other basics there that feel EXACTLY the same as s/he does. Let me say that again. Tell them that EVERYONE else there feels EXACTLY the same that s/he does. Tell them to not be afraid to talk it out with their room mate or others in his flight. They too will feel better knowing that they TOO aren't the only ones feeling this way.
If you want to let him know; my son (Who is class of 2012) is a big and tough football player in high school. Grew up as a military brat. Knew as much about what he was getting into than anyone could. His ALO was also my commanding officer. There was nothing hidden. He knew everything that could be known. He was uber popular in high school. Super athlete. Super 4.0 gpa in IB. Star football player who dated the head cheerleader, etc... He was the most confident applicant to walk into the academy. And he had a really rough time the first few days. Go figure. And so did a lot of others.
Most of this is to help put your mind at ease. But basically, let him know that every single person in that place is feeling exactly the same as him. Hope this helps. Mike.....