It may be more important to establish good eating habits at this age than it is to worry about what he is eating. You have taken a course in Nutrition and know what to offer. The adult decides when the child will eat. A child's liver is not large enough to store glycogen for more than 2 to 3 hours. The child decides whether or how much he will eat.
Every child has a choice at any time food is offered.
He may choose to eat or choose not to eat. Remember that he is the only one that knows how much food he needs. If he doesn't like corn he just doesn't eat corn. You don't need to cook green beans for him. If you start cooking items to please every member of the family you will become a short-order cook.