A chronic cough can be caused by several things...If the cough started as a cold and worsened and there is mucus production (yellow or green) it could be bronchitis. He needs a course of antibiotics. If there is clear or milky mucus it could be an asthmatic cough, particularly this time of year when tree pollen is in the air he could be reacting to it, in other words, the cough could signify allergies. If he has smoked for many years and coughing, or coughing up blood or pinkish mucus it could be a sign of something going on in the lung such as a tumor. Does he have any shortness of breath with the coughing? He should see a doctor to determine the cause of his cough.
I also forgot to ask if there is any burning in the throat. Could be acid reflux or GERD. In any case, he needs to see a DR.
You haven't mentioned if he has seen a DR. If not, he should to at least have a chest X ray. Any back pain? Stomach pain?
Coughing is usually a sign of something irritating airways in my experience. Does he cough all the time or intermittant? Maybe hes allergic to something or possibly asthmatic! Continually coughing I should imagine would make his throat sore. Maybe he should keep a record especially if the coughing occurs in certain situations . He could then take this with him to see his own Dr. Hope this is of some help . I'm sure someone else will reply theres a lot of people on the forum who will know more than me. Take care.