Please stay with the physician who has been treating your son all along and give him or her a chance to clear up this infection. But it still concerns me because he's had it for about six months now. He's tried all types of decongestants and some asthma treatments but nothing works. The doctor says he might grow out of this cough. The cough could be caused by an allergen in the home or outdoor environment.
Smoking can cause allergic reactions and thus the cough.
He also could have allergies to food or smoke from wood fireplaces. You might keep a record of when his cough is worst to see if you can relate it to anything. If he is growing, eats well, plays and enjoys normal activities, does not appear sick or have a fever and the cough doesn't seem to bother him, the doctor may be right in saying he may grow out of it. If possible, have your son seen by an otolaryngologist, or ear, nose and throat specialist. This kind of specialist could rule out allergies.