Coughing up a lot of mucus

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

[Tricia1031]

A related discussion, coughing when I set down was started.