I talked to my mom today. It's definitely irritable bowel. And unfortunately, it's chronic, so he'll have it for the rest of his life and will have to be watched and treated for it. Right now, he's on a mix of dry and wet food and seems to be doing well.
Thanks. :) It's better than some of the other things it could have been. I guess the doctor was actually concerned he might have cancer, because he had the same symptoms as another cat that ended up having cancer. But the biopsies said no cancer for sure.