What Causes Fluid Retention In Ears With Loss Of Hearing?

my son is 8 years old.he had a hearing problem last year due to fluid in his middle ear..i was prescribed mucolite syrup,notril and an eardrop..we also had his audiometry don..he was fine in two months ...since last ten days he is again goin thro the same problem..he is not able to hear low sounds..what should we do..suggest tests and a gud doctor ..