Ducklings can suffer from respiratory illnesses if they're unable to blow their nares (nostrils) out. Does he have access to clean water that's just deep enough for him to dip his bill in and blow out the nares? How old was the duckling when you brought him home? He may not have been able to wash out his nares wherever he was previously (for example, some farm stores may not have waterers that are well-suited to waterfowl, or the water may not be kept clean).
Make sure he can blow his nares out, safely (waterer shouldn't be deep enough for him to climb into). Best of luck!
