I am sorry to hear about Henry's repeated episodes of colitis, with symptoms of bright red, bloody, mucoid stools despite treatment with tylosin and flagyl (metronidazole).
Colitis or inflammation of the large intestine, can be due to stress, a change in diet, parasites, a viral or bacterial infection, inflammatory bowel disease or even a food allergy. Rarely a mass in the colon can be the cause.
Since he has had repeated episodes and seems to feel well a virus is unlikely, and because he is young and symptoms are sporadic a mass is unlikely.
More likely a chronic bacterial infection, parasites, food allergy or sensitivity or immune medicated colitis are behind his symptoms.
I assume that he has had several fresh stool samples submitted to the laboratory and checked for parasites as well as abnormal bacteria. I would also recommend worming him with Panacur (fenbendazole) as a precaution. Whipworms cause sporadic episodes of bloody, mucoid stools, can be difficult to pick up on stools samples, may become endemic in the yard and require multiple wormings and being placed on a monthly wormer to control them.
Has his diet been changed? Some dogs will require a prescription low irritant but high fiber food to control symptoms of colitis long term. Purina Pro Plan Veterinary Diets DCO Dual Fiber Control Dry Dog Food and Royal Canin Veterinary Diet Gastrointestinal Fiber Response are both excellent options for dogs with chronic colitis symptoms.
Some dogs with chronic, sporadic bloody stools require immunosuppressive therapy to quiet the inflammation in their colon. Ideally we would want to rule out infectious causes first and get a firm diagnosis via a biopsy before these sorts of medications are started. Possible drug therapies include steroids (prednisolone, budesonide or dexamethasone), cyclosporine or azathioprine.
At this point I would encourage you to have more testing done to look for abnormal bacteria or parasites and consider multiple wormings and a diet change. If those aren't helpful then biopsies to try to determine the cause behind his symptoms are best.
Best of luck with Henry, please let me know if you have any further questions.