As a plebe, who can barely go outside the walls or do anything, they don't have much need for that $400/month, do they? Maybe he can use it to fly home for holidays. I have to admit, I'm kinda surprised by all of this. ( happily) Are there expenses I don't know about? Do they pay for their club sports/activities out of that? That could definitely get expensive.
Plebes don't get $400/month -- more like $150-200/month depending on if you paid all or part/none of the initial $2,000 deposit (if deposit was not paid or not paid in full, a cadet gets less pay each month Plebe & Yuk year; if paid, they get the full amount -- may be $200/mos Plebe year) . Cadet pay goes up incrementally each year. We paid 3/4 of the deposit with a scholarship (that was late) and a check, so my DS only got $150/month Plebe year; as a Yuk he gets $200 (class of 2018 is getting less than 2017 got as Yuks -- I think it had to do with the iPad requirement or other additional expenses, but basically Yuks are not getting the same amount that prior year Yuks got.) They should catch up by Cow year. I think it goes up ~$100-150/class year.
If your NC is part of a club/sport that has trip sections (out-of-town trips), he may spend some of his pay for additional food. Transportation, etc should be covered by the club/sport. If your NC has to replace a piece of his uniform, buy toiletries, or want additional snacks etc. he will use his pay.
Cadets are given book money at the beginning of each semester (actually well in advance of the semester) and learn early on how and where to buy (or trade) textbooks (Amazon.com, fellow cadets, etc). Any money not used on textbooks is theirs to keep.
Additionally, "top off" is money in the "cadet account" (not personal bank account) that is available to cadets after the successful completion of Plebe year (but in August), and then after each semester. For my DS it's been a few hundred dollars each time. Cadets can take "top off" if there's any available and put it in their personal checking/savings accounts. The way my DS explains it is that if he didn't take "top off" it rolls back into his cadet account (this is the account the Treasurer manages for yearly expenses/uniforms/haircuts & laundry, etc). I don't know if that means a larger amount may be available the next semester. Either way, my cadet looks forward to taking "top off" and putting it into his own account. Here's the treasurer's expense summary for the class of 2019 so that you can see a lot of expenses are upfront during Plebe year, and then larger again Firstie year --
http://www.usma.edu/parents/SiteAssets/EXPENSES CLASS 2019 MEMO.pdf
BTW, the expenses listed above are non-negotiable. If your NC decides to do his own laundry in the barracks, he's still going to be charged the laundry fee.
Your NC may or may not have many expenses besides those I mentioned above, besides trips home, trips for spring break, or trips into NYC with fellow cadets on pass. It all depends on if your NC is a saver or spender.