Hi Fiona, Welcome to HealthcareMagic Forum.
Your kid is suffering from Infant Constipation
. It begins with the transition from breast milk to formula feeds or from solid foods to table foods. Consult a Pediatrician
so that he can rule out Hirschsprung's disease or cystic fibrosis
and follow simple dietary measures, give your baby a daily serving of water in addition to regular feedings. If this is not helping, offer fruit juices and fiber rich feeds.