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Basic reason for stool to become hard is lack of water content in the stool. Give frequent feeds of small quantity of warm water. This should loosen the stool. Ripe banana, grapes and in general all fruits are good for preventing constipation
. You may use fruit juices.Egg and milk are constipatory.
Avoid using any kind of laxatives, as they are habit forming and very difficult to wean later.
Do not force feed your child. This will have exactly the opposite effect of what you want. It will create a hate attitude for food, in the child. Give the child what he likes and when he wants.
Control your anxiety
and don't worry about his weight gain. Nature knows best.