Yes, we had a chicken pox "scare" last winter. As you know, Simon is in the same trial as your son. In our case it was younger brother who came down with a case, despite being vaccinated (was a very mild case in the end, maybe 25 spots or so, I think the vaccine did help for that. His school bus seat-mate had it bad). Simon (also had been vaccinated against chicken-pox) was prescribed anti-virals by the Montreal docs just in case since he was immunocompromized with the steroids, but he did not get any spots. They were mostly worried that he could pass the virus to other immunocompromized children in the hospital during the weekly visit, so he had to wear the heavy-duty mask and be seen in a negative pressure isolation room for a month (cancer ward). But told him to go to school and be in the general public, no problem.
One time when I was a kid I got impetigo on the butt one summer, probably from staying in my wet bathing suit too much. Could it be that?
Hope you get down the the bottom of it ;-} , and that he is feeling ok.
I would suggest you take him to see dermatologist. If is on the skin a dermatologist is your best bet to get correct diagnosis.