My son had roseola; it came with a fever, followed by a rash a day or two later. It didn't seem to itch or bother him. It covered his trunk, had a few spots on his legs and neck, but no spots on his arms. The rash lasted about 5 days.
A year later, my son had a rash on his trunk, arms and legs. I thought it was an allergy to new pajamas, lotion, strawberries, or something else that I couldn't pinpoint. This one was worrisome because it lasted FOREVER. We took him in to one doctor who had no idea and gave us the pat response, "It's probably a virus, call us back if it gets worse." 2 months later, for his 30 month check-up, we saw our regular pediatrician. He knew right away what it was, but I can't remember what he called it. He said it is a common childhood rash with no known cause (he noted that it is not a virus), and it would end around X number of weeks after our visit. Like clockwork, the rash faded when the doctor said it would. It was a total of 5 months of small, slightly raised, reddish bumps. Now, there are a few spots of faded pigment where my son tanned but the spots did not, so I recommend that you are diligent with sunscreen if/when your son's body sees the sun while he has the rash.
Good luck!