Couldn’t hurt. He’ll get plenty of exercise in boot camp, too.
I don’t know if it was called the ASVAB back then, we took a battery of vocational testing that NO one could have studied for. A lot of it was pattern matching and that sort of visual thing.
Back in the 80’s, you could enlist without a high school diploma and, IIRC, you made it through boot camp, they’d send send you to school to earn a GED and then on to AIT. Today’s armed forces are more technical and more selective. Maybe they’re more forgiving and remedial once you’re in, I don’t know. But you gotta get in and make it through basic before you can begin to really go anywhere.
My nephew says there’s an “online preliminary ASVAB” that you can take to see what the test is like. “Well...then take that until you do well on it before taking the real thing,” says I. “Get used to doing some kind of exercise in the morning, too. The better shape you are going in, the easier it will be.”