My 11 Year Old Son Coughs Loudly At Night When

My 11 year old son coughs loudly at night when going to bed. This continues until he falls asleep, and prevents him from sleeping at times. He uses an albuterol inhaler with aerochamber, vicks chest rub, gargles with warm salt water before bed. None of them seem to help. He has nasal drainage that tickles throat and causes coughing to clear it, which irritates throat and chest. He has had a negative skin test for allergies about 3 years ago. I'm at a loss as to what to do to help his night time coughing. We have not given nyquil or similar cough medicines. Any ideas?

Thank you,
Matt