Persistent cough. What to do next?

My husband is 37 years old and works for merchant navy. Last 7-8 years he has been getting cough very often and it doesn't heal soon. It would last for over a month and slowly subside. He smokes not very frequently may be one pack a month. He is overweight and has cholesterol slightly on higher side. He works out well in gym and sweats a lot too. Now since last two months he has severe cough with normal phelgm. we consulted an ENT surgeon who asked for TC DC, ESR, IGE and chest xray.. All reports are normal and with almost one month of medication the cough is still not better... We live in area with lot of dust and pollution... Just too worried with the cough which is not getting better. Please suggest what to do next