Hi, Welcome to HealthcareMagic Forum.
Is he having Bloated abdomen or constipation
associated with this? Is he feeding well? Are you feeding him breast milk or cow's milk?
It is called Melena and there are various causes for it. Kindly get him examined by a Pediatric Gastroenterologist to rule out if he has any anal fissure
or Cow's Milk Protein allergy
.All these factors has to be ruled out. Get a stool analysis and a stool culture done after discussing with his Doctor.