I see your boy is eleven years old. How nice. That's one of the best ages.
The appropriate question is "why".
A cough is a way the body uses to clear out something that is blocking either the trachea (the airway to the lung), or the esophagus (the flesh tube connecting to the stomach.
So one needs to decide if he is clearing the airway or clearing the tube to the digestive tract.
If he is clearing the airway it may be because of as build up of clear sticky mucus. This is often produced in an asthmatic child or adult.
If he is clearing the esophagus, it is due to reflux from the stomach. A little bit of acid coming up.
Children are susceptible to sinus infections, and a "drip" from the sinus will cause a child to continually cough. Often a "spray" of ordinary saline (salt water) into the nasal cavity will clear up the sinus passageway.
It's really difficult to comment on an internet posting board.
The best thing to do is to visit a nurse or a pediatric physician who can perform an in-person evaluation.
You are absolutely right BSML. Possible aspiration of liquid that ended up in the larnyx is what I meant to say...and did not.
You explained things better than I did.
Acid reflux is also a possible cause as acid can reflux up into the larynx and then be aspirated causing the cough. It is impossible however to "clear the esophagus" of anything by coughing. A cough is forcing the air out of the lungs to clear the lungs, bronchi, trachea and possibly the back of the throat of an irritant with a swift contraction of the diaphragm. Since the esophagus goes to the stomach where there is no air, it is impossible to force air through the esophagus with a cough. Furthermore, the stomach lies below the diaphragm, so a contration of the diaphragm would not cause a reaction from it.
Another possible cause of a chronic cough is habit. Children are especially suseptible to developing a habitual cough. If this is the case, a dr (sometimes an allergist/asthmatologist) can give the child other behaviors to employ instead of the annoying cough.
All that being said, it is very important that you take your child to a dr for proper evaluation and diagnosis and treatmnt of the cause of the cough (sometimes there are more than one cause). With school starting, this should be high priority as I am sure that you don't want your son to ridiculed for his cough as kids can be cruel.
I hope that you find answers for you son's cough, and your sanity soon. ;)