9 year old has chronic cough...please help!

My 9 yo son has a chronic cough that keeps getting worse. It started with a cold about 1.5 years ago. It was just a nagging cough initially that never cleared after a cold. I never thought much about it, but now it is a very frequent cough that is annoying for him and everyone around him. It seems the worst before bed time and initially upon waking up. He doesn't cough while sleeping and it seems better during the middle of the day. There are no other symptoms. I'm at a loss as to what is causing this cough. The doctor is no help. And, I want a cure not a mask. Any help is very appreciated. Tweet