There is the potential of velvet here, but did you ever tell us how long you have had this cardinal? The loss of color was most likely from a bacterial infection, though those are often preempted by a parasite of some kind. I didn't see any white dots on him in that second picture, though I wasn't able to see much at all on him aside from the loss of color. Any chance we could get a better quality picture?
I'd also like to ask if he is (or has been) displaying any of these symptoms: Heavy breathing, flashing or scratching, swimming into the flow of a powerhead or return, hiding excessively, lack of appetite.