Brian and Heather have given you great advice.
It sure sounds like hypoclacemia. ( low calcium levels in the body). Heather is right.... If you have been dusting, however, this is unlikely?

If you have not, getting calcium into him is imperative. Weight him and purchase liquid calcium Brian put in the previous link, or get it ( with instructions )from a vet. Like Heather, I'm wondering as well, what brand of calcium are you using, how has it been stored, and how old is it?

Sorry he is sick The pedi soaks will help! Keep at it , we hope he truns the corner.
Lynn