Hello and hope you are doing well.
His symptoms could be due to a post nasal drip, which can cause a chronic irritation in the throat. This could be due to a sinus infection, a cold or a chronic allergy. It could be sinusitis, if it is accompanied by nasal stuffiness and facial pain or pressure. Post nasal drip could also be due to allergens. These are substances like pollen, dust, animal dander, certain foods, insect venoms and certain medications. Exposure to allergens causes a runny nose with a resultant post nasal drip where secretions from the nose go backward into the throat and irritate causing cough. Cough is a protective mechanism by the body as it helps to prevent infections. He will need to consult his primary care physician for proper assessment.
Hope this helped and do keep us posted.
He has allergies. See if he can use a nasel steriod spray, this will help control the nose from dripping so much in the back of his throat. He might also have asthma.