Hello , is he passing stool

Their abdominal muscles (especially the abs) are weak With time the muscle tone increases and they are able to keep the fat and body content inside and then your child will look thin as a school going child looks. It can be due to gas and stool, so if the child is having stomach ache and hard stools then he needs help. For above digestive problems avoid fatty food,  beans, fried food etc. So avoid them. Encourage more liquids. Usually this pot belly goes away by 4th year of age