I'm an asthmatic aspiring boxer. One thing I have found VERY peculiar is that my asthma especially acts up while running. I can swim for days on end with no problem, and use the stationary (and more so the recumbent) bikes with little problem. When I box, however, I barely notice it. I breathe heavily, but never feel the trademark clinching of an attack. It's really weird. Could just be a big psychological thing, who knows (was diagnosed with sports induced asthma and perscribed an albuterol <?> inhaler, but never use it), but it's possible your son may even experience something similar. Not likely, but if it can happen with me, why not him?