UE isn't as prevalent in Cotirnix quail as it is in Bobwhite quail....that having been said, within the last few years it has become more common in Coturnix.How is he doing? @Ingrid_N great posts. The first on the list for Quail is ulcerative enteritis...need streptomycin.
I agree with @NabikiI would try some Nutri-Drench for a quick energy boost.
How old is he?
What are you feeding him?
what does his poop look like?
How tall is his coop/pen/enclosure?
My first guess is that he may have hit his head, but it's hard to say without a lot more information.