Re occuurent cold cough. Sick so easily?

My son is 3 years old. Weight 13 kg. He has been recovered from cough last week. It take his 7-10 days for recovery . Now again since yesterday evening , running nose and cough again started. Why this is happening. He falls sick in every month or two month. This time twice in a month. First he get cold runny nose then wet cough then dry cough and high fever 104-105. Then medicine antibiotics. Every time I need to give because he feels uncomfortable like blocked nose sleep disturbance. My question is why he fall sick so easily?? And every time medicine will make worse effect in his body. Isn't there any permanent cure or any syrup to give him for long time. He is suffering from upper and lower respiratery infection. Which comes and go. My baby has not catch any growth due to this. What to do. So i started giving him septilin syrup only. What could be further diagnosis. CBC is normal. No dengue malaria CRP is normal but esr is high. Esr is 23. Suggest genuine guidance.