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Hi. My name is*****'m sorry to hear about this situation with Walnart. Do you have any idea how old he may be? Is he one that you can handle and would consider taking to the vet's?
That is sad the the neighbor would do this to him. Some people just don't respect animals and I hope that someday karma comes back on them for how they previous treated them. I'm glad to hear that you're looking out for him. As far as weight loss, the simplest thing would be if it was due to intestinal parasitism. I'll be honest and say that this just doesn't happen all that likely with cats. More concerning would be if he would be diabetic, hyperthyroid, be FELV / FIV positive, or if he had some chronic renal issue. If he was a little older, then cancer would be definitely increasing as far as concern. With him being skin and bones, honestly, the best thing that can be done is to get him in a cage for that trip to the vet to have some testing done to see what of the above may be going on. At home, all you can really do there is to continue to provide him with as much nutrition as possible to support him that way. The challenge here most likely will be if any diagnosis can have therapy that can be done. If he's hyperthyroid or diabetic, the treatments require daily handling of him.
His eating all those creatures and being outside could let him have some round worms. They are the ones that can cause weight issues. Tape worms are usually pretty benign. When I'm treating roundworms, pyrantel pamoate is a pretty commonly used medication for them. This may be something that has a product a local pet store.