Suggest Treatment For Congestion And Excessive Mucus Production

My 17 yr. old son is a swimmer. He has been in the pool at least 5 times a week for the last 5 or 6 years. He has seasonal hay fever and an allergy to mold. Within the last 6 months to a year the amount of mucus he produces is alarming. He constantly has to blow his nose or clear his throat. He feels like it has gotten so bad that its affecting his ability to control his breathing while swimming. He is in amazing shape, he eats extremely healthy - we try to stay away from dairy and gluten. He eats primarily organic foods and very little sugar. Would you have any suggestions? Thank you -