The calories, more than the quality of those calories is important now. Chicken soup made with plenty of tender chicken would be helpful. Offer him carbohydrates that are easy to digest and can enter his system quickly. Sports drinks would be a good choice, as would juice. Appropriate foods would be muffins, toast, cereals, and pancakes.
Fevers often increase the requirement for vitamins, including vitamin C and some of the B vitamins.
The use of certain drugs may also interfere with the synthesis and absorption of some vitamins. Therefore, if your son does not generally take a vitamin and mineral supplement, you may want to give him one for the term of his illness. Fluid intake should be liberal. Keep a fresh water bottle by your son's bed, or keep it full of a diluted mixture of sports drink or juice with water. Small amounts of food served at intervals of two to three hours will help him meet his nutritional needs with out overtaxing his digestive system and making him uncomfortable.