This is quite common in dogs. Unless you’re seeing any evidence of hind leg or lower back pain (lameness, difficulty rising, crying) we can put this down to him getting pleasure out of being scratched in that area. From what you’re saying his joints seem to be in good health. Yes….watch his weight!
The area you’re describing is called the lumbar area and this is the most common site for itchiness in dogs. The itchiness can just be a part of his characteristics but it’s important to make sure that he’s not suffering from any skin problems that are causing him to be itchy. We need to consider the following:
So if he has no signs of fleas, no dandruff or hair loss and the skin looks healthy it may just be a part of him. I’d suggest obtaining an oatmeal shampoo (pet outlets) and don’t encourage him by scratching him there.
I hope I’ve been of help.
Kindest regards, Peter
Dr Pete gave you a good medical explanation for possibilities but there are behavior reasons too.
The fact that your dog is backing up to you, staring, and demanding that you scratch him may be a sign that he thinks he can boss you around and 'make' you scratch him on his 'command'.
If that is the case changing how you work with him may be a big help in reducing these demands for attention.
The nothing in life is free program is a good way to change who your dog sees as the leader in your relationship without any violence. You can read about it here
While its nice that you can reach to scratch his back where he can't, it should be your choice to do that not his.
Hope this helps you!