I would look at others his age who are also exercising and see how they are reacting to know if your son is reacting "normally."  Hydration is extremely important, especially if he could have mast cell problems.  You could keep track of his bp and hr, but perhaps an easier way might be to pre-treat him with H1 and H2 histamine blockers, gastrocrom, and  Singulair and see if that helps (if he can tolerate those meds).  It might give you no new information, or it may stop the near fainting episodes.  Definitely discuss this with his doctor, especially any new meds and dosages, and only let him try a new medicine for the first time when he's not going to be exercising.

Heat is such a huge trigger for mast cell disorder patients that I suspect it's the primary problem.  Also, definitely make sure you have emergency meds and epipens at his sports events.

The safest possibility might be to have him do an exercise test in a hospital setting where they would check his bp and hr and could  quickly tend to any possible emergency.  They could also do a tryptase test at a time when he isn't feeling well.

I hope you can find out what is causing his problems and that it is something easily remedied.  Although he isn't fully diagnosed, his doctor might be willing to prescribe him some H2 antihistamines and possibly the other meds just to see if they'll help.  In general, they are relatively benign, unless of course, if he's specifically allergic to one of them.  He sounds very sensible, and that is great!

Thanks Sandi for some more ideas.  I'm not sure he has had potassium checked, certainly not recently anyway. I had a long talk to his PE teachers last week, to at least make them aware that he is having problems, in case anything happens.  It's hard to know what to say to them, because there is nothing concrete, but they at least needed to be alerted to it.  I also wanted them to be ready to be observant of what goes on if he does have any sort of episode.  

He is not an in-shape fit runner, this is compulsory PE sessions and atheltics carnival that everyone is expected to participate in.   As for what's normal, they said in years of teaching in this climate they've only ever had 1 student faint, so he's not so typical. Last year he was having some electrolyte repalcement drinks, on the Dr and dietician's suggestion. Not sure if it made much difference, but he hasn't been having them lately so maybe it did help.  What made me sceptical about that was that he did a school trip late last year camping and kayaking for 5 days. It was physically gruelling, and he drank more water than usual but still not up to the recommended amount, and yet he didn't have migraines at all. He was taking antihistamines, double dose that week as his urticharia was flaring and he was covered in mosquito bites. All in all a very stressful exhausting trip, with no electrolyte replacement drinks (even though I sent him with sachets to make up his own), and all the other stresses, yet no migraines.  Made me think the dehydration wasn't the main factor in his migraines we had thought it was.

I took him for blood tests last week, primarily to check for anaemia as he has been low/borderline before. He is within range. Blood sugar may have been low when he had the latest near-fainting, as he has never been good at eating in the mornings, and this session was before lunch.  However the teacher didn't think he had pushed himself enough to push his blood sugar that low. Who knows.  We just have to keep watching and looking for signs that rule in or out whatever.  The inconsistencies in his reactions is part of why I think a mast cell problem might explain it, the combinations of triggers, plus the success with antihistamines.  But I think we still have a long way to go before it is clear.

good idea Joan, also the grasses running on/ fertilizers etc. or surrounding the track. Ruth I just realized you were the Ruth with the teenaged son we've all talked to before, and yes you are correct to continue to suspect mast cell issues, either with this incident or without. He has a history, I forget it all, but I remember it's long also! Sorry I forgot! There are so many of us, then in switching forums a lot of us including me haven't put faces to our photos yet so it's harder for me to remember who I'm talking to!!!  Years before when masto was not on our radar but I had known environmental/chemical/ and major food allergies. I'd get frustrated when I'd try to get into shape and start my workouts. I'd tell my DH, I swear I'm allergic to exercise! Hopefully not your sons case! The electrolytes don't hurt especially in the heat, he may be whooshing them out also if he'd had something like that it would have helped his blood sugar too! How's the pollen levels where you are right now? More thoughts!

Ruth, the other thing I thought of in the middle of the night was Exercise induced anaphalaxsis( I mutilated the spelling of that so bad spell check doesn't recognize it! Anyway, say your son can have chicken or peanut with no problem. However when put under physical exercise and has something particular it will induce anaphala  .... gotta look it up
anaphylaxis .... ok thats how pub health spells it dang it! So under decent physical exertion some experience anaph after eating! So one must eat a certain amount of hours before exercising.

I have MCAS. I exercise 4 to 5 times per week and ride my bike 4 km to and from work each day, so I am fairly active.  Yet, exercise is clearly one of my triggers because if I overdo it, I will get all kinds of symptoms. including feeling faint and dizzy.  The only time I have ever gone into anaphylaxis was after sprinting for planes and trains.  Just like your son, I did not feel the effects while I was running, but only after I had stopped running and had been sitting for a few minutes.  

Deb gave me advice that I should be sure to warm up and cool down for a little while to avoid the worst symptoms and I have seen an improvement now that I am doing that.  I also take a Gastrocrom 30 mins to 1 hr before exercising.   I never sprint all out anymore and only push myself after a good 20 mins of exercising.  

Perhaps your son can sit out of the more intense activities or do some kind of warm-up beforehand.  

Hopefully pacing will help.  I disagree that it's just a matter of being fit, though that is some of it.  I am quite fit and I still get all kinds of symptoms if I push it, especially in warmer weather.   For me, it was a catch-22 until I figured out that I needed to warm up a lot:  exercise would trigger me so I would not want to exercise, so I would not exercise and became less conditioned.  

Following advice on this forum, I realized that if I did low-intensity exercise for longer (which was easier and did not cause my symptoms to flare) and then began higher intensity 20 or so minutes into it, it was easier to get out there--even if, in the end, I did 20 mins of walking and then 5 mins of jogging.  Now I'm up to much longer jogging times, but I still flare if I push it too hard.    

I don't know if you had luck in finding a diagnosis for your son.  If you need any medical advice, I am in Europe, as well, and it took me several months to find a good mast cell specialist to diagnose me. I am very happy with my current doctor, so message me if you'd like more information.  I would be happy to share it.