When my daughter had Roseola, my pediatrican said "you'll probably want to sell her." So yes, cranky is very much a symptom. Give it a few more days, and if he's not better, take him back to the doctor.
Alternately, he could have fifth disease, which is marked by a low grade fever and a "lacy" rash all over the body. There's nothing to do about it either, and the rash comes and goes. You can google some good images and see if it's like what your son has. My advice is the same, though, wait a few days and if he's not better, take him back to the doc. Good luck.