Same thing happened to my son. It is viral infection now days. Coughin during sleep is bcz of mucus through nasal passin to throat which trigger instense cough during sleeping. In my case, it was for almost more than a month. I took son everywhere but all in vain. Then i went to Sidra Emergency. They said chest is clear but there is congestion in the throat bcz of the mucus passin via nasal.
Only Amoxiciline suspension (Not Augmentine etc) was prescribed and with blessing of Allah, within 3 days his night sleep coughin went away.
I will ask you to take him to any emergency or just take amoxicilline from any pharmacy and give him for 5 days accordin to his age. I also gave him Prospan for cold & Flu three times a day along with that.
If less than one year, take him to emergency.