What Causes Rashes In Groin Area Of Child?

My 2 yr old son has a rash on his groin area starting near the belly button and going just past his buttock. At first they look like little red pimples some bigger Than others and then they dry out and scar but the rash is not going away and it s been about 2 months. I ve stopped using soaps and body wash, changed diaper brands, stopped using baby wipes, changed to a sensitive laundry detergent, etc.