As an MS1/2, I do not believe you get it for a whole 10 months/year. From what I've gotten, I've gotten it just for the school year - nothing more nothing less. Whenever you don't go for a full month (ie: August, September) you get paid per diem.
However, MS3/4 years I believe it's something to that effect (10 months). My understanding is that cadets get paid over the summer months as well - someone here would have more info on that though.
You are correct. DS is a rising MSIV at CLC and has received his stipend this summer.
He should wait before he spends that or seek clarification. Unless something has changed (and I'm no longer connected with USACC), cadets can NOT receive both stipend and summer training pay at the same time. HOWEVER, due to system requirements (ie they can't take 3s/4s out of the system for the summer only), MS3s and 4s will receive both usually and have the stipend taken back.
I can try and dig up a digital copy of the cadet pay info as I just threw away the hard copy I had last week.
You'll get paid when you're in school up to 10 months. If it's not a full month, they'll prorate the stipend.