What Causes Severe Cough Along With Clear Mucus Discharge?

Hello, My husband has been coughing up a lot of clear mucus. He has done this for years but is has gotten a lot worse over the past year. The mucus is clear and thick and enough at a time to fill the bottom of a glass up. This mainly happens after he eats. He has taken several types of pills but done have worked. He also has to blow his nose a lot with the same clear mucus. He has been going to a Thoat, Nose and Ear Specialist and he ran the light down his nose and throat and said all was clear. He also had a chest xray and it was clear. What could this be? This Dr. said he wante to send him to a Gastrologist.. Can you help us? Thanks