A BMI of 18.88 would be pretty normal for a 13 year old if not a bit on the lower side already. Usually we consider a BMI between around 18/19 and around 25 to be that of a normal weight person. If it is below theis, we would recommend to eat more, if ti above this we would recommend to eat less. Considering that we're talking about a 13 year old, perhaps it would be better if he/she ate a bit more to supply his body with the necessary nutritional needs.