Is your son on breast milk or formula milk?
If he is breastfed, you need not give him additional water.
If he is on formula milk, see to that you are giving him in the right dilution as adding more milk powder with less water may result in hard stools. You may give him water additionally. Also see to that he consumes adequate quantity of milk.
But breastmilk exclusively till 6 months of age is the best for your baby.
Please visit your Pediatrician
to check if some anal fissures are present. If fissures are seen, due to pain, he might hold the stools when it reaches the fissured area. This in turn causes impaction
and hardening of stools. So a vicious cycle will be set in. In such a case he will need stool softeners and gels to be applied in the fissure.
You may give him fruit juices and concentrated sugar solution at home.
If after 3 days he does not pass stools, do visit your Pediatrician.