Incessant chronic cough in 9 year old for 3 months

[shastri20032003]

Hi Friends.My 9 year old son has been having a  dry cough without any wheezing since the past 3 months now. He has been coughing non stop from morning to night but when he sleeps he does not cough in the night. He does complain of a slight chest pain and besides that there seem to be no other symptoms....I suspect the chest pain may be due to the incessant coughing
We have taken him to a few docters who have diagnosed him with mild asthma and he has been onto oral medications and inhaled dry powder(TURBUHALER) since the past few months but the cough only seems to get worse
Have done a food allergy test and have started the food elimination diet. Also trying to keep him away from the standard allergens like dust mite etc
What I immediately need to know is the following
What is the most probable cause of his coughing? I know this is a patient to patient forum but any
Is there any medication to give him a temporary relief from the coughing?(cough suppressants) as my heart goes out seeing him cough so much?
Do I need to do any more tests like CAT scan,allergy skin test, routine pulmonary test, methacoline broncial challange test etc?
My next plan of action is to see both a Paediatrician who specialises in asthma and allergy and also to visit a Respiratory c who is a cough and chest specialist
Any response would be extremely welcome as I am very worried
Regards