Why does my 1 year old keep getting bronchitis?

I have taken him to his pediatrician at least twice a month for his coughing and wheezing. She keeps telling that it is bronchitis. She prescribes an antibiotic and tells me to keep using the nebulizer twice a day. He is still coughing. He coughs as soon as you take him off the nebulizer. Has anyone else had this problem? What else can I do?