If he's not eating, not pooping, and is sitting hunched up grinding teeth, that's called GI stasis. Any painful condition can cause it, but often it will be from some digestive upset. I will usually try some baby gas drops(simethicone, dosing in link below), 3 doses an hour apart, and often that will get them back to eating and acting normally again. If my rabbit isn't starting to get back to normal after that and/or there are signs of bloat or true fecal diarrhea(not just mushy cecals), bun hasn't eaten for more than 12-24 hours, or the rabbits condition deteriorates, then it is an emergency and the rabbit needs to be seen by a rabbit vet immediately.
So if this is the case with your rabbit, you could try phoning the vet back and say your rabbit has stopped eating for more than 12 hours, and is hunched and tooth grinding and that it's an emergency. If they still won't fit your rabbit in, then try and find a vet that will today, even if you have to travel a bit to get to one. Or find an emergency clinic that has a vet experienced with rabbits.
If you still can't find a vet that will fit your rabbit in today or tomorrow at the latest, then I would continue with the gas drops, then maybe add a little unflavored pedialyte to replace lost electrolytes and restore blood sugar levels. You just don't want to be syringing in very much in case it is a bloated stomach or a complete intestinal blockage, as this will add more volume and pressure on the affected organs, which could prove fatal.
But if by tomorrow you still can't get your rabbit in, if there's no bloat occurring then I would start considering starting syringe feeds. It's not recommended in most cases as it can cause serious complications if a rabbit has a complete blockage or bloat, but it's also not good for a rabbit to continue to go without eating for an extended period.
https://rabbit.org/gastrointestinal-stasis-the-silent-killer-2/
https://rabbit.org/vet-listings/