Welcome to EmpowHer! You have every reason to be concerned about your son possibly having Kawasaki's Disease. In Roseola, children get a high fever, then the rash will break once the fever is either gone or decreased. Conjunctivitis, vomiting, and diarrhea are NOT symptoms of Roseola, unless your son was given antibiotics in which case he could have diarrhea as a side effect.
How long has he had this fever for? If it's been longer than 3 days-- take him back to his doctor or the ER. I had a similar situation with my 1 year old and it turned out to be Roseola-- in her case, the rash lasted an entire 10 days after the fever broke. So, what I'm trying to say is: go with your gut! If you feel like this may not be Roseola, take him back--let them run proper tests. A simple blood test with elevated white blood cells can be an indication of Kawasaki's. In Roseola, white cells remain at their normal levels.
Please keep us posted,