Have you tried an ent? Ears nose and throat doctor?
Might be from allergies, we had to give our son zyrtex to help worked in a week, pollen is really bad here
My 14 month old is on zertec too. Otherwise his nose doesn't stop running.
Try Claritin if the problem is sneezing and Zyrtec if the problem is coughing and phlegm in the lungs. Also see the doctor about getting a nose spray with a cortisone style base. Chicken soup is great but sometimes you need something better. Do you take him to a pediatric allergist? A pediatrician? Or just your family doctor?