wheezing

My son is 5 years old. When he was born he stayed in the N.I.C.U. for a week because he had problems breathing. 1 year later he was admitted to the hospital for another 4 days( reason was croup). Aproximately 1 year after that i took him to the doctor because of his wheezing and he was put on nebulizer breathing threatments. Later on that year he was put on the breathing threatments because of his wheezing, now at the age of 5 his wheezing has returned. the doctors has never diagnosed him with asthma. Could it possbily be something else more serious?