For 2 years 11 kg weight is alright. If your child weight and height are increasing then there is no need to worry about the cough. If you have family history of allergic cough your child also may get affected. For this the main stay of treatment is prevention. Diagnosing allergies is very difficult. so you have to observe the environment of your child when ever he is getting cough. You have to identify the triggers like dust, construction work, soft toys, pets, other children with cough and cold, cold food items, pollen, climate etc and avoid those things. I will come down with growing age may be after 5 years of age it may come down. Hope your doubts are cleared.