What Causes Cough When Lying Down On Right Side?

THANK YOU,MY HUSBAND HAS HAD A COUGH FOR 10 YRS,HE COUGH WHEN HE LAYS DOWN,ALSO WHEN HE LAYS ON HIS RIGHT SIDE. THE DR GAVE HIM ADVAIR PUFFER 500MG.IT MAKES HIM COUGH MORE...HIS HEART IS GREAT THE DR.SAID,HE GETS PHEMONIA ONCE A YR. HES NOT A SMOKER,THANK YOU...