Since your son was born 6 weeks early, he is more likely to develop asthma. However what you have described could be asthma or reactive airways disease (RAD).
Infections like a cold or pneumonia can cause inflammation of the airways of the lungs. Usually when this occurs, your son may cough when he is active or laughs. This inflammation can cause wheezing. After the infection is gone, it is possible for the inflammation to linger. This inflammation can last for several weeks. Sometimes this inflammation may linger for 3 to 6 months. Eventually the inflammation will go away, and then the wheezing will stop. This is called RAD and behaves a lot like asthma. This inflammation often clears more quickly when it is treated with an inhaled steroid, like the Pulmicort Respules
Sorry about the double post