Fur where curly tail touches back

[semni]

My Shiba has a tightly curled tail that rests on the lower portion of his back. Ever since he was young his tail has matted down the fur where it touches the lower part of his back. However, after a bath the fur on his back always perked back up again. He's now 8 years old and I've noticed that he's actually losing his fur in this area. Rather than having a fluffy undercoat and a rich overcoat, he just has stubby short hair. I've parted the fur, looked at his skin in this area, and it looks normal. Does anybody have any idea what might be going on? I have attached some photos as evidence.