Help finding problem to bad rectal bleeding and severe abdominal pain!

[A0L0W]

My finance is a 39 yr old Caucasian male. He's had rectal bleeding for the past few months but it's been small amounts like spots on the tissue. About 3 weeks ago all at once the bleeding increased to the point where the whole toilet bowl was bright red and he has terrible lower left abdominal pain that shoots into his lower back. He went to the emergency room originally, they did a CT scan but found nothing and recommended a colonoscopy. We went and he had that done and we were told everything looked fine. After seeking out a second opinion, we did a second CT scan which showed Diverticulosis but no signs of inflammation. The Dr said this shouldn't cause excruciating pain or the amount of bleeding he's been having. His lab results also showed lower than average Hemoglobin levels. We have an appointment made to get a Flex Sig procedure done next week, but if that comes up without any answers we don't know what to do next. Both the GI specialists that we saw are completely stumped. I'm looking to see if anyone else has had these symptoms and hopefully if anyone has a cause or treatment that we could look into. He's been in so much pain he can barely move around and cannot work, and he's not the type of guy that will let manageable pain slow him down and he never misses work for anything! Please help!