A rash can have a lot of causes, including an allergic reaction. He might be having an allergic reaction to fleas, grass or another irritant.
I suggest that you bathe him well with a good shampoo. Then dry him well. See if this helps with the redness. If it's a contact allergy, it will give him some relief.
You should also give your vet a call and see if benadryl would be a good idea to combat any allergic reaction that he might be having. My vet recommends 1 mg per lb of dog given every 8-12 hours.
If this does not go away on its own and a vet visit will be needed.
My biggest concerns is that this is demodectic mange or ringworm so you will want to see the vet to rule these out.
Let me know if you still have questions.