If you have liquid Corid, you can give a couple of drops of the undiluted to the very sick chick twice a day to boost it. Very ill chicks may not drink enough of the mixture.
I am glad that you have received advice on treating for coccidiosis. That would be my first guess. Corid is the best treatment, since Sulmet and other sulfa drugs are either off the market or only available with a vet prescription. Sulfa is an antibiotic, but Corid (amprollium) is not. Corid is...