My son's weight is only 12.3 kg his age is 5 years how to increase wait,any healthy tonic please suggest

A. By the age of five years a child should be somewhere around 18 kg so it’s a big gap between the correct weight and the weight that your child is maintaining as of now so it would be good to check with your doctor to know what is the problem and accordingly they can help you with right supplements and diet that you should follow for the child