Why Does My Dog Rub His Backside Along The Floor?

He does it to scratch an itch. You may want to check him for worms.

Contrary to what many think (worms) that is usually a sign they need their anal glands expressed and are trying to do the job themselves (generally unsuccessfully). They have glands just inside that they used before they were domesticated to mark their territory. They no longer do so and most small dogs and many larger ones need to have them manually done on a regular basis to avoid infection and impaction. Your vet can do this and many groomer's do so as well (and a bath is useful since it SMELLS and it sticks around).