Personally, am almost perfect BMI, at 51 (5’10 and 68-70kg) have been a circuit and fatigue training instructor in my young working life.
He is both overweight, and unfit, for the majority of his time here.
But, most, most importantly, he carries bad fat, if you bulk on Omega3 rich fats you enliven the cells by nourishing/providing the Oxylipins (et al) this is the balm that coats cells and enables oxygen conductance.
Budwig called this “the electron cloud” (Flax oil has the highest surplus electrons) which provides sufficient Electrons that the cells in the blood don’t need to share and surprise, surprise, they test for Omega3 blood index to ascertain survival likelihood after major cardiac events, blood tests show cells de-couple, similar to grounding/earthing studies (where we pick up needed electrons from the earths bounty).
Get him off the fried Chicken (whatever his weakness is) and onto Salmon and Flax, the boy would be transformed.
IMO, he needs to drop around 4-8 lbs of fat, or, transfer that to Omega3 rich fat, best both (lose about a stone of fat, resupplying 2/3 with healthful fats).
This is the core, IMO, to why he gasses so readily, and when challenged with a hard leg to leg challenge, he wilts.
As a kid, had a mate who was brilliant at Cross Country, but he was a short, barrel chested kid, but he could run and run and run. He was obsessed with Tuna.