His weight is below the 5th percentile for his age. It is frustrating when your child has no interest in food. Is he drinking milk? 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) in place of milk so he is getting calories and protein to grow. Suggest giving him the milkshakes or smoothies in place of milk at each meal. Suggest give him daily a children’s chewable multi-vitamin due to him eating habits. Try engaging him in the kitchen more. Have him help you with planning and preparation of food, and setting the table. Hopefully this will increase his interest in food. The most important is to relax at meals. If he will not eat a certain food not eat too much, do not make a fuss over it. Let it go and enjoy the meal. Suggest asking the Expert Child Behavior forum for additional tips. Hoped this helped you.
Your post caught eye. I'm searching online for the same issue. I have twin daughters that are 9 and also weigh 42 lbs. They are about the same height as your son also. Have you been able to find any answers?