I caught chicken pox as an adult and it started out with ONE bump the night before I woke up with them all over my body. Fever would be present with chicken pox as well. I doubt they would pop up with a one here today, a few tomorrow and a few more after that. Chicken pox looks like little red mosquito bites at first, but they then look like little blisters. And, my chicken pox started on my left arm. It looked like a pimple, which I attempted to pop, but nothing came out. The next morning, I was covered with them all over my body! Yes, chicken pox is still out there.
As for Roseola, MOST children have gotten Roseola by the time they are 2 years old ~ it's unlikely your son has Roseola. I have yet to meet a mother whose child got Roseola after age 2. Though I am not saying it is impossible, I do believe it would be rare. Roseola starts out with a high fever and the fever lasts for several days. No other symptoms would be present ~ normally it's just a high fever and I'm talking about 101 and higher. When my first born had Roseola, her fever spiked to 104. ONLY when the fever breaks does the tell-tale rash come out and I have 3 children, I saw the rash mostly on their stomach, back, and chest and neck but not on the face or arms. Once the rash breaks out, the illness is over and you don't catch Roseola twice. It's viral.
And, going to the E.R. is a waste of time. I am not sure why so many folks run to the E.R. for non critical situations. A couple of bumps is not a life threatening situation. In my opinion.