Our daughter has been flaring this week. What type of preventative is your son taking? Perhaps it's time for a preventative med change. . .our daughter was always well-controlled on the nonsteroidal inhaler: Intal. Our son, however, an allergic asthmatic (versus the daughter who is viral) needed a steroidal inhaler: Beclovent to control his symptoms. He also had/has to use an allergy medication to help control the symptoms.
If your son has only been diagnosed within the past year or so, perhaps the physicians haven't yet gotten a handle on the triggers. Sometimes it helps to keep a diary of the episodes to give clues to the trigger(s).
In any event, good luck--I have lived with this since our daughter was 16 months and nearly died (she's now approaching 13). Our son became symptomatic at 5 (after a T&A) and he's now 15. If you need to chat about it or looking for more advice, I'd be happy to share our knowledge AND there are a good many RNs on this board, too! A second opinion never hurts! Best wishes for a quick turnaround. . .