It certainly sounds like classic colitis with the red blood and mucous, jelly-like consistency of the stool. An endoscope is the way to diagnose it, but you can see if he responds to medication and diet changes as well. Usually sulfasalazine and metronidazole are used to treat colitis and adding fiber to the diet will show resolution of the symptoms. Also, labwork and xrays are normal with this problem.
To answer your questions, a fecal test would rule out parasites. There are no blood tests, unfortunatley, to diagnose cancer in pets.
Other possibilities would mostly be diet related - if he ate a bone, a rawhide treat, table food, etc. can cause these signs. Irritable bowel is a remote possibility but usually doesn't come on suddenly and the patient has chronic vomiting and diarrhea.
Dr. Debbie
Veterinarian
Full time practicing companion animal veterinarian.