Bleeding from the anus accompanied with severe pain might be a sign of very serious condition. Your father might be experiencing a condition called acute abdomen. It can be caused by various reasons, but what is important is that he has to go to Emergency service as soon as possible. He even might have to have a surgery done. Whether this should be necessary can not be told without further tests (physical exam, laboratory tests, X-ray and/or abdominal ultrasound). Seeing a doctor immediately is highly recommendable.