It is common for children to have a larger belly as they enter their pre-teen years due to an increase in height and weight. This is because of the growth hormone that stimulates bone, muscle and fat development during this age. Additionally, if your 11 year old son does not get enough exercise or eats too much unhealthy food, this can cause him to gain weight around his abdomen area. To help keep your son's midsection trim, it's important to encourage physical activity like walking or riding a bike and limit eating snacks high in sugar or fats. It may also be beneficial to speak with a doctor about nutrition tips tailored specifically for your son’s needs.