It could be nothing but a boy testing you, but I'm glad you are considering other possibilities.
I am reminded of an episode of Mystery Diagnosis (These are true stories, not made up TV shows) that I saw not long back. I'm not diagnosing, but you may ask your doctor about it. There was a young girl who would lose her energy upon exertion. The doctors told her she was chubby, lose 20 pounds. She did and it didn't help. The teacher took the class on a walk, and she was fine until she had to walk uphill. She had to stop, breathe, and it took a lot of effort to go on. She passed out when she got back to the school. The principal was an EMT and said her heart was racing. When they went to the Statue of Liberty, she couldn't make it up the steps. She finally went to a doctor who couldn't figure out what it was, but sent her immediately to a specialist who said she had Pulmonary Hypertension. So, if that sounds like your son, I'd ask the doctor about it.
I'm not a nut job that tries to diagnose my kids off tv shows, but they do sometimes give me things to think about. (Not all doctors know about all illnesses.) I hope it helps your son.
B.