From your measurements, his weight is below the 5th percentile for his age. It is frustrating when children of this age start to be 'picky eaters'. Suggest making milkshakes (milk, ice cream, and fresh fruit or frozen) or yogurt smoothies (yogurt, fresh fruit, and frozen fruit) and give it to him in place of milk (in a sippy cup) for meals and snacks for additional calories/protein to help increase his weight. Engage him in the kitchen more. Children love to be a part of making food. Have him help you with preparation of food, make the milkshakes and setting the table. Hopefully this will increase his interest in other types of food. The most important thing is to RELAX. You do not want mealtimes to be a battle of the wits between the your son and you. 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. 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 weeks. Hope this helped you.