Monya:
The rash you describe certainly sounds viral, a diagnosis which would "fit" with your son's fever, other symptoms, and normal blood count. Measles is, of course, a virus too, though it has a particular pattern which many modern physicians are poor at recognizing because the condition is so rare nowadays. The color change and "blending" you describe also fit with a virus. 8 days is by no means out of line for this kind of rash. (I assume he's not taking penicillin or some toehr drug which could cause an allergy.)
Which virus it is doesn't matter much. Viruses go away without much help.
My advice is this: If your son is still "sick" (high fever, etc.), perhaps you should visit the Children's Hospital for another opinion. If not, wau a few more days. I bet the rash just disappears leaving a little superficial peeling for a while and not much else.
Good luck. Who says being a parent is easy!
Dr. R