Added: I just noticed something on cnn.com on heatstroke affecting more young football players these days. At the end, they mention that 50 years ago, people were not acclimated to air-conditioning, so their bodies had a higher tolerance for heat, so it seems like the growing number of incidents has made coaches and trainers more aware. Thankfully, your son survived his brush with danger, and hopefully made a lot more people aware.
http://www.cnn.com/2009/HEALTH/08/21/heat.football.practi...
My heart completely goes out to you. This sounds like heat stroke (or hyperthermia) rather than just overheating. (And it's not the same as fever, so don't feel bad that you couldn't find a thermometer.) I remember reading up on this a few years ago when a pro football player, a promising young athlete, died during summer training. (Hot day, lots of padding, his body just couldn't cool itself fast enough and it came on QUICKLY.) Heat stroke can come on VERY suddenly, it's life-threatening, and warrants a trip to the emergency room. (I don't mean to make you feel bad for not calling an ambulance, you obviously did a WONDERFUL job of caring for him and deserve a lot of credit. But if anyone reading this experiences the same thing, CALL 911!)
If indeed his core body temperature did rise too high, maybe there was some damage to his internal organs that still hasn't finished healing. (I didn't research this too in depth - according the entry for heat stroke on the Mayo clinic and Wikipedia websites,it looks like once you get them past the immediate danger, they do OK.
Wikipedia did say that "After efects may include sensitivity to heat." But they had no further explanation. Try reading everything you can find on hyperthermia and after-effects.)
The Mayo clinic said something about some people being more suseptible to heat stroke for various reasons. (Beth, below, posted something about that.)
Although August heat, padding, and excercise is probably enough for anyone, especially if he wasn't super well-hydrated before starting.
You didn't say whether or not he's still doing football or other sports. I'm assuming he's not feeling up to it, but I'd take him out of all strenuous activities, and not resume anything until several months after all the stomach pains and headaches STOP. (Normally, I wouldn't be this paranoid, but he's STILL feeling bad WEEKS after the incident. And it sounded like it was a very close call.)
You said something about "dealing with the pain and going on with doing activities" - but maybe you just meant activities like regular school stuff. I would talk to his teachers and INSIST that he not be allowed to do strenuous, outdoor activities like P.E. until he's back to 100%. (Yes, I know - maybe I'm a little paranoid.)
Tell them that if he gets very exhausted during the school day, take him to the nurse's clinic make him lie down for a little while.)
Also talk to the school nurse and let her know that he's recovering from a recent episode of heatstoke and to be a little extra vigilant.
If he's exhausted all the time, that's probably the best thing for him. If his body really is still recovering, rest is best. (If he's truly so exhausted that he's willing to stay in bed all day, I might even consider letting him stay home from school, if that's even POSSIBLE with everything else going on in your life. Not skipping school every day, but... you'll have to go with your gut instinct. If he's battling exhaustion and just trying to get through the pain, it's not likely that he's *learning* anything that day.)
It's heart-breaking to see your child in pain. At least it will force him to slow down and not exert himself, though.
Do you give him anything for the headaches? Maybe you can ask the doc if it's OK to at least give him an Over-the-counter pain killer, like excedrin or ibuprofen.
(I'm not a doctor, but I don't think it has anything to do with the added stress of your illness or anxiety or bowels or constipation or chiropractic or whatever. It sounds like he was fine until the episode at football practice, his internal core body temperature got way too high that day, *maybe* doing some damage to his internal organs, and he's still not completely recovered from that.)
Good luck with the specialist - I think that (and LOTS of bedrest) are your best bet.