What can cause a 7 year old child coughs (mild) for about 2 months now?

See pediatrician. You should have your child see his pediatrician for a definite diagnosis, as many things can cause a persistent chronic cough - depending on the severity of the symptoms and other symptoms. It could be allergies, asthma, reflux, a virus, or other infection - to name some common causes.
Long list. A number of conditions can cause prolonged cough in children. Asthma, sinus infection, foreign body aspiration etc. The child has to be seen to have a proper evaluation.