The thing that comes to my mind is e. cuniculi, a parasite that causes symptoms that are sometimes referred to by vets as a “bunny stroke”. It is treatable with Panacur (fenbendazole) if you get to your vet asap. I think you should keep the bunny warm and comfy meanwhile, sick buns often get hypothermic.
(This may not be e cuniculi, I’m just guessing here, you will need a vet to diagnose him.)
e cuniculi is a very common parasite in bunnies and most buns never get ill from it, but if the bun has been subject to stress from f ex a new environment, a recent illness etc, it may trigger a “bunny-stroke”. It can give many different neurological symptoms.
Your other buns can be treated too as a preventive measure, it’s a de-wormer called panacur that you give for 28 consequitive days.
Your sick bun may need supportive measures too like sub Q fluids, pain-meds, a gut motility drug etc.
Please keep us updated.