More information would be good what kind of a rash , is it raised ,red blister looking? It sounds as if it may be an allergy. What has he been doing , any sport, has he had new pants , have you changed laundry liquid. Is he well any fever , any other symptoms is the rash itchy, or painful .?
Are you in a warm climate, could it be heat rash?
If it is an allergy I think a Hydrocortisone cream will help ..you can get one OTC but also go to your doctor who may prescribe some thing stronger ..