How To Improve Appetite In A Kid?

Hi my son is 16 months old. He had chicken pox in Jan 2015. And now he is fine. But since then he has stopped eating the food that he used to like. Now I have to run after him but still he does not eat much . He takes a bite of the food item and then after chewing the food for the minute he throws out the food from his mouth. His weight is 9kgs. Dr has advised me to give him cypon syrup for weight gain. Pls tell me what should I do.