Try feeding him food of high nutritional value so he gets as much protein and calories as he
needs.
For example, having a protein source each meal --- meat / fish or cheese or yogurt; with a carbohydrate --- bread, pasta, rice, cereal; and a fat ---- pat of butter or mayonnaise if you made a sandwich. Good protein sources are meats, chicken, fish, beef, pork, etc.; and dairy products such as milk, yogurt, cheeses, pudding, and ice cream. It is good he eats pork and chicken.