If it comes to food, parents' responsibility is to prepare healthy and nutritious meals for their children. On the other hand, children have responsibility to eat it!
Children have an automatic ability to regulate how much food they need for growth. Forcing children to finish their meals may disturb that mechanism and promote overeating and obesity. If your son is not hungry for lunch, he will have a bigger appetite for dinner. In between meals, structure nutritious snacks to provide him with the necessary nutrients. Always offer fruits and vegetables with every meal and snack and expect that they are eaten. Offer fluids only after meals so that he doe snot fill up on them and prepare the vegetables in various ways. Don't give up if your son turns his nose on one kind of a vegetable. Some children are extremely picky and may need to be exposed to the same food many times even before they attempt to try it.