Does he chew it. It's kind of a hard to reach spot but if there is a will. It sort of looks like the spot on my GSD's rump when I first rescued her. She would worry at the spot until it was bleeding. Vet said it was a skin irritation causing her to chew it. Nothing worked until once in absolute frustration I applied some Avon Skin so Soft hand lotion and she seemed relieved by it. I began applying it daily and the skin cleared up and she grew hair back. It was funny because up until she passed away, I could never get used to her having hair on her rump, we had lived with it for so long before I could fix it, the hair seemed unnatural, almost like a toupee.
Anyway. best of luck with finding out what it si. I would check with a vet because it could be caused by an allergy or a skin irritation. Rule out all the causes.
My littlest one is experiencing the exact same thing right now. Definitely get yours to the vet, but for us, it's one of two things: Seasonal Flank Alopecia (which is our original diagnosis) or thyroid problems.
I am having thyroid panels done at the end of the month to rule it out completely, but from all things I've read ours is seasonal flank alopecia, which is caused in part by not having enough sunlight through the winter months.