Wow, this sounds like what my DD12 has. It started this past June with a cough that wouldn't go away and she's been battling it ever since. She's been on several inhalers as well as the zithromax/prednisone route (she hated the prednisone, it caused some nasty side effects). It helped a bit, but not much. We took her to a pulmonologist earlier this month and she upped her dosage of the advair, so we are hoping that helps.
Honestly, it's been very frustrating becuase it seems everything they try does not work! To make matters worse, she is a gymnast and the coughing is always much worse when she is at practice. The combination of chalk dust, hot gym, and intense physical activity really set her off.
I don't know if your son's condition is similar, but as for my DD, the pulmonogist is hoping that this increased dosage of the inhaler will help and that she will eventually go down to smaller and smaller dosages and eventually be weaned off entirely. She was never diagnosed with asthma, per se. I kept hearing things such as 'asthma-type allergy', 'bronchitis', and 'excercise-induced asthma'.
This has been a shock to us because she has always been so healthy. It's been so frustrating for her as well becuase she had been doing so well at gymnastics (she placed first at state last year). Then right after our WDW trip this April she fractured her foot and now this bronchitis stuff...it has really set her back.
I hope your son is able to get over this quickly!