Overnight Vet for Bloody Stool

My poor boy suddenly had horrible bloody diarrhea today and has been refusing to eat. We are so lucky that our ER could see him immediately. Unfortunately they can only do basic care there, but at least he's somewhere safe with professionals. They couldn't see or feel anything inherently alarming with a basic physical exam, but of course bloody stool should ALWAYS be taken seriously. He's staying overnight where they'll attempt to hydrate and get some form of food in him, and will be transported to a better equipped facility in the morning for diagnostics. Really hoping it's something easily treatable, he's only 5 years old and we've had him since he was a baby. He's always been so healthy and happy. This is the first time I've EVER left an animal at a vet overnight and it's so hard...