How long since Elvis stopped eating?
How often is he passing the diarrhoea?
Is he bright and alert? Moving around ok and seems interested?
Thanks for the extra information.
18 to 20 hours without eating is unfortunately bad news for a bunny, Their guts rely on constant movement and activity to keep going and once they have stopped for more than about 12 hours, they can be very difficult to get going again.
The diarrhoea may well be caused by and infection such as coccidia, can be down to eating something toxic, or an inflammation of the intestines and imbalance of the bacteria in the caecum/large intestine of the rabbit. Rabbits rely very highly on the activity of bugs to complete the process of digestion for them and if these are not in the proportions that they should be then they can produce runny faeces.
Having had diarrhoea originally and then not eating of drinking, it is likely that Elvis is feeling pretty dehydrated and poorly - these guys feel pain very severely and so if the guts are hurting, he will feel bad. A combination of the dehydration and any discomfort then stops them eating, so they get into the vicious cycle and become really poorly.
My best advice would be to get him to vet as soon as you possible can for an exam and some supportive fluids and drugs to try to get him through this. It may be that with rehydration and pain relief that he may feel more like eating, or they can syringe a high fibre mixture to try to get the guts back on track. He sounds very poorly an although treatment is not guaranteed to save him, it is probably his best chance.
If you can't get him to the vet straight away, the best that you can do is to have him indoors, keep him warm and try to get some warm water into him with a syringe, followed by feeding with a syringe of fruit/veg based baby food, pellets mashed with hot water or even a little live yoghurt. Try little and often and stop of he's not swallowing the food as it can go down the wrong way and into the lungs.
I hope this helps, and that he can see a vet soon,