Allergic Dry Cough, Relapse While Playing With Pet Dog. Treatment?

My son is 5 years old and suffering from allergic dry cough. some times before, his doctor advised him inhaler and we continued it for almost two years. But after 2 long swimming sessions, his problem subdued. Two days back, he played for 5 minutes with a pet dog near our house. and since then his cough relapsed. today, he is coughing continuously. Though we tried a cough syrup but in no use. please suggest something that can be useful.