Every kid fall sick frequently with cough and cold every year, if he is able to recover well in a short span then there is nothing to worry. Few tips to get an better immune system a. Don't self medicate with antibiotics for flu b. Give high protein diet through food c.Give variety of seasonfruits and vegetables    as it contents lots of mineral and     antioxidants. d. Promote physical activity and avoid junk. P.S. - there is no immunity booster medicine, every product in market is just to make money.