I'd probably wait and see. Rashes tend to be viral and so there's little that can be done about them. If he's not currently running a fever and doesn't seem to be bothered by the rash or have any other symptoms of illness (eating and sleeping well), I'd wait. But that's me.
Mom to two amazing DDs ('07 & '09) and a fur baby.
Gluten free since Nov '11 after non-celiac gluten sensitive diagnosis. Have had great improvement or total elimination of: migraines, bloating/distention, heartburn, cystic acne, canker sores, bleeding gums, eczema on elbows, dry skin and scalp, muscle cramps, PMS, hair loss, heart palpitations, fatigue. I'm amazed.