What's the height, weight and age of the child? You usually dont have to worry about lids not eating, they will eat when they are hungry. If it goes for an extended period of time I would maybe consult his pediatrician. Usually kids 1 1/2 to about 6 years old can have a nutritional supplement called Ensure, it has lot of vitamins, nutrients and healthy fats
My daughter was in the 80th percentile for height but only the 20th percentile for weight the first few years of her life. She was perfectly healthy just skinny. The best person to ask is your brother's pediatrician next checkup. If your brother is loosing weight than than you should make an appointment to get a medical opinion.