Hi You take him back to the doctor. 5 and half month with cough gasping for air may be worrisome. Have you immunised the baby? Is there any chest in drawing or without cough is he struggling for breath? Is phlegm thick or thin in consistency? These questions need to be answered to reach a diagnosis. You keep him well hydrated with frequent breastfeeding. Give him humidified air. Put nasal saline drops. He may have post nasal drip so you can keep him on side during sleep but in strict supervision .