My Father In Law Recently Discovered Blood Inside His Stool.

My father in law recently discovered blood inside his stool. He has had blood outside his stool before but he mentioned that this was different from any thing he has experienced. It started on Friday after eating a portion of McDonald's hamburger then a regular dinner. It was not until that night he discovered the blood inside his stool then 3 to 4 loose stools after that. He is 79 going on 80 years old in June. Any suggestions. He did experience pain with these stools.