Talk with your son's pediatrician about his rate of weight gain over the last 6-12 months and the rate at which he is growing. His trend on the growth curve is far more important than the number of pounds he weighs on any given day.
I'd say whether you're talking about helping a child with obesity or helping a child with underweight, advice is often the same. Remember that you, as a parent, are the provider of healthy food. Your child is the eater of food. This division of responsibility is essential for helping a child learn to crave, eat, and grow on a diverse diet full of whole grains, veggies, fruit, lean meats or protein and dairy or soy products. Force feeding never works, of course, and neither do struggles around eating for increasing or decreasing what a child eats. So, if your child is struggling with overweight, or underweight, the recommendations are similar: Offer healthy diverse whole foods every day, sit with your child during meal time, chose mini-meals for snack foods rather than processed food.
For parents of of children who are underweight, I often also suggest thinking about offering at least one calorie DENSE food at every meal--something nutritious like avocado (full of great fats and calories), cooking veggies in olive oil, or keeping a pint of heavy cream in the fridge so when making pasta or cereal you can add 1 tablespoon of the cream to their dish to fortify calories.
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