What specialists is he seeing? Has he had allergy testing? I would take him to an allergist and also an ENT. I'm assuming he has a pulmonary specialist if he had bronchoscopy. He should have a CT of his sinuses, if he hasn't already. I had an asthma attack in February. I never had asthma before then. It has taken nearly 4 months for them to figure out that I have chronic sinusitis and that I was having constant drainage into my chest, resulting in back to back bronchitis and eventually what appeared to be an asthma attack. It is possible that I do have asthma, but the sinusitis is what induced it. If no one has looked at the possibility, then you need to find a doctor who will. My pulmonary doctor was the one who actually ordered my CT scan. I wasn't seeing an ENT because I didn't really think I had sinus problems.
Oh, also make whoever his doctor is switch him to xopenex right away. It is much easier on the heart that albuterol. I think it may be an insurance issue, because it costs more, but it should be standard practice of kids! I was put on it because I had a bad reaction to the nebulized albuterol. I couldn't believe the difference! It worked right away and lasted twice as long!
There are a lot of moms on this board who have children with asthma, I'm sure you'll get plenty of good advice here! Good luck!