How can I cure my baby's constipation?

It depends... This answer depends on whether the baby is breastfed, how is the baby feeding, is the baby gaining weight well, is the baby eating solid foods, is the baby's abdomen distended? How often does the child have a bm? Is the bm soft? Are they rock hard? Does the baby cry when he has a bm? The answers to these questions are necessary to determine true constipation in a baby which is quite rare.
Fruit juice. You can add fruit juice to baby's diet. Preferable apple, white grape, or pear juice. Start with an ounce a day at first. Sometimes trial and error is required and patience.