Hello, my name isXXXXX and I have over 20 years of experience as a veterinarian.
I am sorry to hear that Tye isn't gaining weight and has loose stools.
Diarrhea can be due to stress, a change in diet, parasites, inflammatory bowel disease, infiltrative cancers like lymphoma or even a food allergy. But it can also be related to metabolic organ disease (such kidney or liver disease or diabetes) due to organ wastes irritating the gut. Rarely a mass in the colon can be the cause.
Ideally because he is an older guy he should be seen by a veterinarian and have a physical examination and some testing done. I realize that may be very difficult to do because he is feral.
I would recommend taking his stool in to your veterinarian for a parasite check as a place to start. Beyond that if there is bright red blood and mucous in his stools then that indicates large intestinal disease, whereas tarry blood and watery stools with no mucous indicate small intestinal disease.
Because he's an outdoor fellow I would worm him whether parasite eggs or cysts are present as they are a common cause of diarrhea and are only shed intermittently.
When diarrhea is present that changes the bacteria flora in the gut so sometimes antibiotics do help, and so do probiotics. You might wish to add Flortiflora or Benebac to his food to add beneficial bacteria back to his intestinal tract.
You might wish to try feeding a bland diet of 2/3 boiled white, skinless chicken), all fats and juices drained off mixed with 1/3 boiled, plain white rice. Feed several small meals a day. Add 1 to 2 tablespoon(s) of canned pumpkin to each meal if you are seeing mucous in his stool as fiber helps soothe an irritated colon. If that diet seems to help you might wish to feed a prescription food called Purina OM, which is easy to digest and has extra fiber which may be a perfect long term food for him.
Please let me know if you have any further questions.