You can take him for a spirometry test which will assess his current lung conditions as well as help to diagnose diseases like Chronic Obstructive Pulmonary Disease. Chronic Obstructive Pulmonary Disease (COPD) is a condition in a patient's airway gets blocked leading to a chronic cough. Although this is not a defining factor, if either you or your spouse smokes, there is a higher possibility that your son may be suffering from COPD.
There is also the possibility of Cough variant asthma. You mentioned that your son coughs a lot at night, which is extremely common with people suffering from this condition. If it is a dry cough that is often accompanied by a wheezing sound, it would be wise to take him in for a test called methacholine challenge which is used to diagnose this condition.
answered by M W