My GF little girl had something very similar only a few weeks ago. She's a bit older than your DS, but the GP put it down to her virus. Hers moved all around her body but they were a bit longer lasting.

However, random moving rashes are concerning if you ask me, a chat to your GP probably wouldn't hurt. Especially as it's been over a week. Viral rashes don't usually last more than a couple of days.