It's also very normal for toddlers to get a mild rash in response to a number of viral infections.

A little friend of DD's was sent home from family daycare last week with a mild rash and a temp, her parents were told "non specific viral illness". Sure enough DD came out with it the next day (with a slight runny nose). It was gone a few days later.

Apparently toddlers get rashes for some viral infections that don't cause rashes in adults. If it's very light and not bothering him I would bet that's what it is. Hope it goes away soon