Jen, I'm so sorry to hear that Jasper isn't improving. How long has he been on the pred? I know it took Starr at least several days to show an improvement but he started out on higher doseage and he was also on a food for allergies.
I also noticed that when Starr would eat wet food that his diarrhea would become worse. My vet said it's because his colon couldn't absorb the liquid like it should so I kept Starr on dry food only until he started his herbal supplements. I sure hope that it isn't cancer and that Jasper will start feeling well again soon. He continues to remain in my thoughts and prayers.