As long as he is steadily growing, no matter how slowly, is what matters. 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.