Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear about Stinky.
Of course you are concerned about him having blood in his stool.
How is he feeling otherwise?
Is he bright, alert and playful, eating and drinking well?
If so this may be related to a change in diet or gastrointestinal parasites, which are quite common in cats. I understand that his stool may have been checked for worms but it is more common for protozoal parasites to cause bleeding and those can be difficult to pick up on a single fecal check, sometimes we need to check several.
If you are also feeding a different food or treats than before and if it was an abrupt change that too can cause diarrhea, sometimes with blood.
Other causes for blood in the stool can be impacted or infected anal glands, a rectal polyp or mass or colitis which is inflammation of the colon. Colitis can be caused by a food sensitivity or allergy or a primary autoimmune (body attacks itself type response). In some cats the colon doesn't move ingesta as it should, becomes flaccid and enlarged, and cats end up with large firm stools that they have trouble passing. This is called megacolon.
I know that your fellow has seen a veterinarian once but if this is a recurrent problem he needs another recheck. Make sure his anal glands are checked, a fresh stool sample is submitted to the laboratory to look for parasites or abnormal bacteria, and that an ultrasound is done to evaluate his colon.
In the meantime to soothe his irritated gastrointestinal tract I recommend a bland diet of 2/3 minced, boiled, white, skinless chicken and 1/3 boiled white rice. Add 1 tablespoon of canned pumpkin (not pie filling, just pumpkin) to each meal to add fiber which can soothe the colon. This is easy to digest and should soothe his intestines.
Once he's feeling well then start mixing in his regular food, adding a little more regular and less bland at each meal. It should take a week to get him converted back to his regular food. If the bland diet and increased pumpkin fiber helps you may wish to continue adding pumpkin to his food long term.
However if he is feeling poorly, not eating well or playful I recommend he be seen immediately tomorrow morning by your veterinarian if he seems stable, or tonight by an emergency clinic if he seems to be going downhill quickly, especially if he begins vomiting.
Please let me know if you have any further questions.