I would suggest you keeping careful observation of your son for the next 48 hours. Sometimes the effects of exposure are delayed. The usual manifestation is vomiting, diarrhea, and difficulty breathing.
There isn't much you can do but give it some time to work its way out of the body. Of course if the breathing becomes very difficult, you can supply oxygen and that will help.
Usually the initial reaction is pretty quick, and then you feel like crap for a day or so. From what you have described, it sounds like your son had the initial reaction and recovered.
It is difficult to speculate on the concentration of gas in a room, but you are starting out with a little over 20000 PPM. You need some special equipment to actually measure the concentration in air, but 30 minutes of exposure causing a reaction suggests that the concentration was high.
The general rule is that if you can detect the chlorine dioxide odor, it is time to ventilate the area.