Suggest Treatment For Constipation In A Child

Hello doctor my son is 4 months and 1 week old I started giving him wheat cerelac of nestle since last 10 days I feed cerelac in a bottle by adding extra water since past few days he is passing his stool in 2-3 days and he is suffering from constipation before he passed his stool one time a day and his stool was also hard. What should I do?