My feral cat is losing weight fast but has a healthy appetite. Why?

Hello. Weight loss of that amount is, unfortunately, going to require some investigation. If the cat’s feeding routine has basically remained unchanged, he has been dewormed....then we need to be looking for other, more serious problems. A thorough physical exam, bloodwork and possibly x-rays. He will likely need sedation for all of this, so presenting him to the vet in a humane trap that he can be sedated while still in the trap is the safest method for all involved. Reasons for dramatic weight loss could include diabetes, intestinal diseases, kidney or liver issues, cancer, etc. I deal with feral cats all the time and it is relatively simple to sedate them through the trap in order to do any procedures necessary. Good luck