It sounds to me like the school did some sort of deferment for your son while he was on leave.
Your son could get an in school deferment now, if he is taking a course load that is at least half time (he doesn’t have to be formally matriculated). But if you can pay the loan off, I suggest contacting the servicer to get a payoff amount. Get it paid in full right away, and you will have one less thing to worry about.
I wish your son (and you) the best.