My first piece of advice is to give him the singulair in the evening, if your dr/pharmacist didn't already advise you to do that, I have been on it myself for over 10 years, and never was to that it should be taken in the evening until about 5 yrs ago, studies actually show that if you take it in the morning, you're at higher risk of ear infections and bronchitis, and I would get bronchitis 1-2 times a year when i took it in the morning, since it switched the time i take it, i have only had bronchitis maybe 2 times in the past 5 yrs, but i also work in a healthcare facility around all those germs.
Secondly, my son had bronchitis when he was about 14 months old and seemed as though he never got over it, and when he was 17 months, we ended up in the ER with shortness of breath, and resp. retractions( you could see the skin at his throat caving in he was struggling to breathe that much) they did numerous nebulizer treatments and he seemed to be improving. Anyway, make a long story short, he was sent home with a prescrip. for an inhlaer and to follow up with his MD, who then prescribed neb treatments, and what a difference! he had them twice a day for a week and only uses them as needed now. they still are unsure of what caused it but the neb treatments work, see if you can get a used neb machine if you do end up going that route, i only ended up paying about $30 for his, the ins. paid the rest it was close to $300