Suggest Treatment For Persistent Cough And Sneezing In A Child

My son is 4 and a half, about 20kg in weight. He is down with mild cough along with sneezing for the past 3 days. I have been giving him s-mucolite. From yesterday he has also started on a runny nose. I was wondering whether I should start sinerest or maxtra? Or anything else you recommend. He tends to catch a cold of this kind frequently, so my doctor tells me he is allergic and that I should avoid dusty areas... What would be your suggestion. There is no fever though.