His weight is below the 5th percentile and his height is on the 50th percentile for his age. Yogurt and egg usually digest well regardless of illness. They are protein sources and need to be eaten daily. Is he drinking milk? That will give him protein and you can add ice cream to it as a milkshake (milk, fruit, and ice cream blended) or yogurt smoothies (yogurt, fresh fruit, and frozen fruit) for additional calories and protein.
Make sure he is eating a protein food each meal (meat, fish, cheese, or yogurt). Some tips for additional calories: If he likes pasta, then add meat or cream sauces to the pasta instead of plain tomato sauce or plain pasta. Add dry milk to oatmeal, mashed potatoes, gravies, creamed vegetables, and creamed soups (add additional meats to soups). Use all fat dairy products and no low fat foods so he gets the calories he needs.
Mealtimes can be a battlefield when children do not have an interest to eating or eat very little. It becomes the battle of the wits between the child and the parents. Remember meals need to be joyous for parents and the child. If he will not eat a certain food, do not make a fuss over it. Give him some other food alternatives and let him eat what he wants. Then let it go and enjoy the meal. Slowly introduce new foods, and if he does not want it, do not make a fuss over it. Let it go and enjoy the meal. Try again in a few months. Hoped this helped you.
He also passes undigested food in the stools.