thanks for your query at HCM
cough, post nasal discharge
might be due to sinusitis or pharyngitis
1 give him plenty of water to drink that will decrease throat irritation
and decrease cough.
2 you can give him piriton that will decrease cough and nasal discharge and will make him comfortable for sleeping.
3 give syrup paracetamol
15 milligram per kilogram three times a day if fever more than 100 f
4 if symptoms are not decreasing visit doctor and he might need start on antibiotics