Hi guys. I'd like to chime in here. I spent 10 years in the Army. Got out as a Captain. I was Ranger, Airborne, Air Assault, Pathfinder and Jungle Expert.
Have your boy focus on pushups, situps and the two mile run. The U.S. Army website posts the standards for the PT test. He needs to do everything he can to max it. Promotions will come easier - actually, most of Army life will be far easier if you can max the test.
As far as humping a ruck goes, you'll never really do it much in basic.
What MOS is he going in as? That may make a difference as to what he needs to be able to do.
Coach, I realize this is probably a little weird for you to hear someone on this forum recommend situps, but, well, you know... it's the gubmint.
You might recommend some strategies that would improve the number of situps you can do without actually doing situps... Does that make sense?
Also, your feet are held down so work with a partner or a sofa or something.
Find someone in the gym with an Army shirt on and have them critique your pushup form.
Hope that helps. Have your boy PM me if he needs more. I'm happy to help.