Hello.. I Have A 3 Year Old Son. His Weight

Hello.. I have a 3 year old son. His weight is 12 kg currently. He was around 10 kgs when he was one year old. Since then there has not been much growth in his weight and height. He seems to be the tiniest in the class. He always says that he is not hungry and does not want to eat. I have been suggested to give him Cypon syrup 2.5 ml for 1 month once in the night. Will it help?