Hello, this is Dr. Linda. I hope I can help with your question today.
I am so sorry for your concerns about your cat. Ingestion of even small amounts of the Asiatic lily can indeed result in damage to the kidneys & possibly even death. Your quick action to get him to the vet for decomtamination ( vomiting & charcoal ) & IV fluids will undoubtedly minimize any significant damage to your kitties kidneys.
Is your cat still eating? This is a very important indicator that all is still OK. I would have no problems with you taking him home tonight if there has been no vomiting. Your vet may even place some additional fluids under the skin ( subcutaneously ), to continue to flush the kidneys overnight.
If your cat is still not vomiting in the morning, has urinated in the litter box, & is eating, he will probably make a full recovery. It is not a bad idea to continue the fluids again in the morning, & perhaps reevaluate the kidney values as well, but if he remains clinically normal, then he will be fine.
I hope this has addressed your concerns. If you have any additional questions or concerns, please do not hesitate to let me know.