This could be several things. It could be minor or more significant. It is not normal to have a cough of such severity for so long. I would suggest seeing a Pediatric Pulmonologist and getting a more complete evaluation. This may include a CT scan of the sinuses and other tests. Only by finding the cause can you take steps to control this prolonged cough.
i had a similar problem as an adult - massive amounts of mucus/phlegm in the evenings - could not sleep until about 2am most nights - too busy coughing up stuff or waking up choking. Short answer for me was food allergies to wheat, gluten, soy. I would have a nice wheat based pasta dinner between 5 and 6pm and the mucous would start shortly after and last for up to 8 hours (dependent on volume of asllergen consumed). What are you feeing your child for dinner - I'd look into doing an elimination process to try to identify if some foods are the cause. I found my GPs were not very useful - tried to treat the symptoms with antihitamine and steroids but it made the stuff thicker and more choking. Hope your answer turns out to be so simple. PS wheat and soy are in lots of things - eh sausages, sauces, ... haev to read labels carfeully. Best of luck
MOST LIKELY AT THIS AGE IT ISNT A FOOD ALLERGY,MY DAUGHTER HAD A CHRONIC COUGH WITH EXCESSIVE PHLEGM FOR OVER 5 MONTHS-WAS TREATED FOR ASTHMA AND SINUS AND THATS IT-GOT SO BAD OTHER PROBLEMS STRTED LIKE BLOOD IN STOOL AND MUCUS-TOOK HER TO LEBONHEUR CHILDREND HOSPITAL IN MEMPHIS WHERE SHE STAYED ALL TOGETHER OVER 2 WEEKS,THEY TESTED HER WHOOPING COUGH,TB,CYCTIC FIBROISIS,DONE A UPPER A LOWER GI,LUNG SCOPE,YOU NAME IT-DIAGONOSED WITH SEVER ACID REFLUX(GERD)WHICH WILL MAKE YOU HAVE A BAD COUGH,SINUS DISEASE AND A INFECTED ESOPHAGUS WHICH WAS MAINLY FROM ALL THE STEROIDS THE OTHER DOCTORS FROM HOME GAVE HER TO TREAT ASTMA AND TO FIND OUT SHE DOESNT EVEN ASTHMA-HER BLOOD PRESSURE BECAME HIGH BECAUSE OF THESE UNNECCESARY MEDS AND GAINED ALOT OF WEIGHT-SHE JUST HAD SINUS SURGERY TO REMOVE TO PUS FILLED CISTS AND UNBLOCK HER 100 PERCENT BLOCKED SINUS-DOING ALOT BETTER BUT LIKE I SAID HERS STARTED ALSO WITH THAT COUGH SO PLEASE GET A SECOND OPION AND GOOD LUCK-LITTLEVICK0