I'm happy that you have asked me to assist you with your child and hope that you can benefit from my many years of helping parents like you.
Skin rashes due to viruses are VERY common in childre and especially this time of year. Right now we are seeing roseola, hand-foot-mouth disease, and various other rash viruses as well as some stomach bugs that cause rashes. That is likely what he had and now that the rash is gone he is over it and not contagious.
You will not be able to find the exact virus that he had as there are well over 100 rash viruses that humans can catch and in our lifetime we usually have 4-6 of them as kids. The great thing is that we immunize against all the bad ones (measles, chickenpox, etc.) and there is no reason to worry about identifying the others like the one he just had.
I see you are still offline but if you have any questions just let me know.