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. He will need to consult his primary care physician for proper clinical assessment. In the meantime for relief from his symptoms he could do warm salt gargles and take OTC antihistamines.
Hope this helped and do keep us posted.
My only issue with this answer is that it happens daily about 8 times a day when he dips. That's the only time he clears his throat and produces this mucous.