Cat keeps having diarrhea

Hello i (22F) just recently adopted a cat who is around 5 years old, he has been having bad diarrhea and there has been some blood in his stool from time to time. I have tried changing his food to Tiki cat because it is supposed to be a healthier brand but it doesn’t seem to be helping. I just took him to the vet on Friday for a check up and for his diarrhea issues. The vet said he was healthy and everything looks and sounds good, she did give him an antibiotic for the blood in his stool and wanted a stool sample to test for parasites. Well fast forward to today they called me back with the results and he does not have parasites. I am at a loss and don’t know what to do to help my kitty. Any advice would be appreciated.