my Persian cat has had severe diarrhoea today, he's a bit lethargic too. the vet won't be open for another 12 hours, PLEASE HELP!!! I tried calling the vet for advice but no answer.
Electrolyte water is probably your best bet in the short term. Some cats respond to plain, canned pumpkin but neither the water nor the pumpkin will cure underlying reasons for the diarrhea. Many human OTC compounds for diarrhea have other ingredients in them which may not be good for cats.
If your cat seems very bad off, see if an emergency vet is available or even call around and see if another vet will take an emergency.