We hear the occaisional horror story of children dropping on the field because they had an unknown heart condition. That doesn't mean this is the trouble with your son, but I would definately tell the pediatrician what happened and pester him for a full cardiac workup just to make sure that isn't the issue. It may be his asthma as well. Just keep pestering the doctors. Also, have your son slow down a little until you know what is going on. Don't have him stop all together, but don't let him push to shortness of breath. He likely knows what it feels like just as it starts, so talk with him about stopping before it happens.
Follow your gut, take him in for some cardiac tests. They need to do a dopplar too, to make sure everything is structurally the way it is expected. Then at least, if everything comes back normal, you know you have ticked that off your list of possibilities.
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