Lactose intolerance usually produces gas and diarrhea. Why is your son on soy milk? Soy milk does not contain lactose, and is often used by people who cannot digest milk, which does have lactose. Constipation can be treated with a high fiber diet - lots of fruit (especially apples with the peel), vegetables, and whole grain bread and cereal. Make sure he gets plenty of exercise. Ask a pediatrician about giving him fiber supplements.