Pembo,
My son (6.5) had this a few weeks ago. First it appeared on his legs down to his ankles. Then it went away after a few days and reappeared on the back of his arms only. No fever, no itching or any other symptoms. Could be a heat rash. If he should run a fever with the rash, I'd have him seen, but I'd wait it out too to watch it go away.
Did he sit in a wet bathing suit? After the rash(s) went away, we spent the day at the beach and my son didn't want to change out of his suit and got a rash right where you're son has one.
Sounds like he had contact with something irritating his skin that he wore or sat on.