What you described could be a rectal prolapse or hemorrhoid. However, it is always best to determine what it is after a paediatrician has examined him. Kindly look into his diet, if his stools are hard, give fruits and vegetables to help it pass easily. If having diarrhea, let the cause be treated. Till you see the Doctor, gently push it back in if it comes out (that is if it stays out/does not return on its own). Further treatment will depend on what is found when it is examined.