extreme coughing and gagging

my boyfriend has a chronic cough and gags alot, he says it's so bad he feels like he will black out from a coughing episode.it is worse in the morning while still in bed and seems to be worse after lying down,he has been a sheetrock hanger for 30 years and suspected asbestos in the lungs:however x-rays were taken and showed his lungs to be clear.his lungs also sound clear.he also chokes on food easily.he does cough up mucus but it's clear to white and hasn't been bloody.he has had this problem for at least 10 years.it seems to be getting worse all the time.I hope you can give us some info that will help or tell us where to go or what to check next. he is also diabetic,but other than that is in fairly good health.please answer as soon as possible,
                                           thank you,
                                           belinda

[Missy2001]

Has he been tested for GERD?  This can cause extreme coughing.  Also, is he taking any medication that could cause the coughing?