My ds had/has a persistent nighttime cough for over 3 months now and only gets worse when he has a cold.
He has been diagnosed with asthma, but Im not certain it is this as the ventolin does not always stop it.
I have also placed a vapouriser in his room, again this does not always stop the cough but does ease it.
I don't know if you have heard of this trick either, but it does work, especially if they have a cold. Place V!ck$ vapour rub on the soles of their feet, cover feet with socks when sending to bed. It doesn't always work with the persistent cough we are talking about here.... but if your little one has a cold I guarantee it will work a treat !!!!
Sorry I don't have many suggestions as we are still trying to find out what is causing his coughing too.
Good luck !
Shell, DS 23/1/06 & DD 11/9/07 WA