Hi D.:
I'm a grandma now but both my kids came down with Fifth's Disease aka Slap Cheek Syndrom when they were 8 & 4. Their cheeks were firey red, they ran low fevers and were not especially sickly through it, even to the point of their doctor saying that if they continued to not show effects from the disease, I should go ahead and send them to school. He did not feel that this was a particularly contagious disease, in fact it wasn't seen all that much. That would have been in 1984. So I continued to send them to school and damn if a bunch of kids throughout the school suddenly came down it. I had some parents chewing me out big time but hey the pediatrician had been clear and the other kids weren't all that sick either.
It does not surprise me that your son is feeling poorly. That makes sense just because he is younger. Indeed this is the fifth disease to be identified in the rubella family. I don't remember that it lasted all that long, maybe a total of 4 - 5 days. But their cheeks really stood out, not just rosy, screaming red. If the doc has given you a diagnosis then I would say quit worrying, give your son comfort with cool baths and tylenol for the fever. Fluids obviously as much as he will drink. I can tell you from experience that my kids came down with all the weird, never heard of, no one else every had, bunch of diseases. And often, when they did this, they looked to be very, very, seriously ill. I was an absolute nervous wreck. I haven't run into another mother or grandmother who can say these same things but I can tell you that there are things out there that sound silly, the diagnosis makes you wonder if the doc knows anything, and my big question was always "how did my kids get this and no one else has?" It is perplexing. If you have a really good pediatrician, I would say relax but I also was a very careful mother who did observe my kids so I could go into the doc with the info I knew he would need. There were times when I felt I needed to press the doc harder or at one time, I did need to make a change in docs quickly. So be observant, know the typical things that your son does, i.e. is he fever prone, ear issues, throat issues, times of year when he is worse, any triggers, that sort of stuff.