He would have graduated from high school in 1970 or 1971 and, assuming a student deferment for college, would have missed the opportunity to serve. By 1975, the military was negotiating with ROTC new-grads to forgive the commitment to provide a commission and in return they forgave the tuition; a very sweet deal. Remember, at that point in Nixon's shut down the officer pipeline was still full and we stopped losing our young 2nd lieutenants and pilots.
I suppose you could make a case for his enlistment right after high school but that would ignore the realities of college deferments taken by the majority of his classmates. It is an approach that would backfire for sure.
Mine was something like 327, which, at the time, was too high to be drafted. I stayed in college, but I know at least one classmate who volunteered, after his birth date came up in the low numbers. Maybe someone else can be more specific about the draft policy at the time, but this is how I remember it.