Children are very good at determining how much they need to eat. They know how to listen to their bodily cues. If he says he is full you shouldn't force him to eat more. This only tells him not to listen to his own body and to overeat, which could lead to weight problems later on. Make sure he gets a snack after school. Also, remember that your stomach is only about the size of your fist. That means that your son's stomach is the size of his fist, which at 8 years old is probably still pretty small. The stomach can stretch, but it shouldn't get used to being overfull. Your son is still a child, he shouldn't be forced to eat what an adult thinks is appropriate since we are used to eating larger portions. As long as the food your son is eating is healthy, as long as he's still growing and is otherwise healthy, I wouldn't worry too much. Kids go through phases where their bodies don't need as much food. It's normal, and as soon as he hits a growth spurt he'll be eating more. Give him a multivitamin and a DHA supplement every day if you're concerned about his nutrition. And also make sure he isn't filling up on liquids right before meals since that will make him feel full.