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Since your doctor has already examined him and found nothing wrong, there is nothing to worry about your son's heart.
Constipation is very common in children and it is usually caused by not drinking enough fluids, not eating enough fiber, or having a diet high in constipating foods (milk, cheese, banannas, etc.).
Children with constipation usually either have very large hard stools or hard pellet like stools. Bowel movements are also infrequent and painful.
Treatments for childhood constipation usually includes stool softeners and changing the child to a less constipating diet, with more fiber and fluids.
Stool softeners are often used for 4-6 months and then slowly decreased to prevent reoccurances.
Good choices of fluids include apple and prune juice and perhaps switching older children to soy milk. Reading food labels can help you to find high fiber foods.