Breathing issues can also be adult onset (or undiagnosed) asthma, which can get worse as you age AND exercise--mostly with your mouth open, and breathe too hard. It’s happened before, and it’ll happen again, to multiple people. It’s tough to believe unless it happens to you; the answers from medical people are often incorrect. Learn to breathe through your nose. It helps. And breathe through your nose while exercising. He was trying to breathe harder, and that’s often when asthma rears its ugly head. I know; it happened to me. 30 years later, after finding my asthma migrated from “exercise induced” to everyday activities, I’ve found the solution--learning to tolerate a bit of air hunger, and breathing through the nose. Look up the Buteyko Method.