Need detailed history

What is the exact weight and height? We have to plot it on a chart to know. This is a very common problem at this age and is not dangerous usually. The child will gain height and weight now in his teenage spurt after 13 years. Till then he may stay thin because that is his body type at this age. Also if mother or father were thin at this age, he is likely to be thin. No need to worry. Just visit a pediatrician. Don't listen to lay persons. Consult a pediatrician

Give high energy food like adding ghee/oil to food; have protein like fish, egg, peanuts.

Food is the best treatment. Kindly add fats in enhancing calories. Ghee / butter in most of the dishes. Give sprouts which are rich in protein.