Hello, I Am Just Trying To See Why My 4

Hello, I am just trying to see why my 4 year old still has a cough that sounds like he has lots of cold in him, starting from October. He has thick white mucus coming out of his most. He is not coughing so much that he can't get any sleep. He can still sleep-no coughing at night. He is not miserable, nor does he have a fever. The allergy test shows negative. First year in Preschool