What Causes Rashes On The Left Side Of Face Of An Infant?

I have a 5 month old who had a rash on his face from his temple on his left side of his face all the way down his jaw line around the chin and up the other cheek straight to the other temple .. Isn't rough to the touch but is appearing dry. I'm assuming teething but he rubs it all night long but doesn't seem to bug him while he is awake.