Stress? Allergies? Has anything changed as of the development of this bald spot? Diet, your daily routine, etc etc. It's probably a hot spot, and I'd get some bag balm like AP said. But it could also be due to other things. You can always dose with benadryl to keep him from itching it, but benadryl isn't a fix, it just masks things until you figure out what it actually is.
Try giving the dog a bath with some mild soap, rinsing and drying very well. That way you know it's not something on the fur. Good luck!