I wonder if he volunteered for the draft. I was 19 in 1973 and nobody in my high school class came close to getting drafted. One of my high school classmate pulled the number 5 in the lottery and he was not drafted. Things were a lot different in 1973 versus 1967 etc.
By that time the services had only two priorities, separating thousands out and desperately trying to find room for those who wanted to stay in. The guards and reserves could only take so many.