I served 12 years in the army, starting at 16.
As you can see you have received a mountain of advice, a lot of it guesswork and supposition.
Get professional advice. If he cant be bothered to to that, he has probably chosen the wrong career.
Do not get him a padded Bergen. I cannot even guess how many hundreds of miles I ran with one. It did nothing for my upper body strength and knackered my knees.
Physical training methods and equipment have moved on so much in the last few years. He really needs to talk to someone who knows what they are talking about.
If he needs convincing, get him to message me on here.
A guy I grew up with became SAS. I visited him on the base in Hereford. I was also military at the time. There were also 3 SAS guys on my army diving course in Plymouth. I met several during my 12 years. They are the most unassuming guys I have met.
Steve