Like other signs of mental illness, this sign can have several causes. One likely cause is depression combined with your child's nervousness and up-tightness. Eating is something to do and a way of dealing with his nervous energy. Eating far too much is somewhat like a natural tranquilizer that makes the young person a little drowsy and calmer.
Another cause is part of an eating disorder called bulimia and is related to the last sign. Your child's weight likely is about right for his height and age; but nonetheless, he vomits to control his weight. After eating, he purges or gets rid of what he has just eaten. As if trying to make up for the purging, he eats huge amounts of food in one sitting. This is not just overeating. In one sitting, your child may eat enough food to serve as normal meals for ten or twenty people. Keeping food away from him or locking up the food usually does little good and may make matters worse. Your youngster needs specialized help immediately.