Cough since 1 Month. Clear the cough!

My husband had a viral fever 1.5 months ago which included fever, cold, cough this lasted for a week and back to normal but the wet cough didn't go even after using a Expectorants for over two weeks , later he got a rtpcr(negative) check up and X-ray done which says normal. The Dr thoroughly checked and said the airways and chest are very clear and diagnosed as mild bronchitis and gave Relent, Rem CC and BIOCOF BR Syrup for a week it reduced slightly and again the cough is back This Cough mainly starts during the evening or if the weather is cold. Hardly there is any cough during the sleep and it's mild cough Which is ON & OFF in the evenings , Please suggest what can be done to totally clear the cough