It sounds to me as though you may be better off seeing a new vet! Often coccidea can't be seen in the fecal float test, and it sounds worth a try putting Stewie on a course of Albon & Metronidazole (aka flagyl) to hopefully get rid of coccidea, or giardia if that's what he could have. Also Kaeopectate (I'm pretty sure) has been reformulated, and is now dangerous to give cats. Another possibility is Stewie is allergic to some ingredient in the food...but this often causes itching.