Is he busy or just not into me?

So, I (19, F) met this guy (19, M) 2-3 weeks before he had to go for a military academy. I liked him instantly because he looked good and judging his energy it was kinda like mine, similar tastes etc.

So we start talking and its progressing well, I find myself in a pursuers role (which i dont mind because hey whats wrong in letting a person know exactly how you feel) and we're talking sweet, he's liking my stories, commenting on my posts, he's even talking about how he'd love to receive my letters.

After joining, all of a sudden, he tells me that he doesnt want anything serious at all. Instead of freaking out I ask, whats the matter and he tells me that he had been aiming for the this academy since middle school and these 3 years of training mean everything to him etc so he doesnt want me to get hurt and doesnt want to get in trouble for receiving letters from a girl (trouble here is some extra drills by seniors for fun so as to "earn" the letter). I say I understand and tell him we can just test the waters and see where this goes.

He continues to have his phone in the academy for the initial 3ish days and calls me once from there and then the usual texts and all, whenever he gets the time.

But, as soon as the phone gets submitted, hes vanished? I mean, its been 3 weeks and I havent heard from him (they have landlines for the cadets to use for calling). Now, I wouldve happily assumed that he got really busy and all, but a few of my classmates also joined the same course, and they keep calling me to tell stories and ask about me... and he hasnt.

I know its not THAT long of a timeframe, like just 3 weeks right? But my mind does question: is he just being very ambitious towards his life there or is he just not into me or both?

TLDR; havent heard from a guy i had been talking to, making me wonder if hes not into me as deeply as i thought.