A 4-year-old boy presents with a history of constipation since the age of 6 months. His stools, produced every 3 to 4 days, are described as large and hard. His growth velocity is normal. Physical examination is normal; rectal examination reveals a large ampulla, poor sphincter tone but present anal wink, and stool in the rectal vault. The plain film of his abdomen is shown. Which of the following is the most appropriate next step in the management of this child?