My yorkie has a patch right above his tail which he likes to scratch (usually against the bottom of the sofa). Defo not fleas or mites. Is it worth cleaning the area with Hibiscrub or a medicated shampoo maybe before we go to the vet? I believe the breed can be prone to skin complaints? We inherited him a year or so ago as an adult, he goes to the groomer regularly for clips but not noticed this before.
If it's a casual rub/scratch it might simply be that it feels good (i say this because you describe as "which he likes to scratch" which sounds quite mild), but if a more concentrated, regular or vigorous scratching of a small area (which or without the area becoming inflamed) it could be a 'hot spot'.
I'd take him to the vet to have his anal glands checked, to be sure, and I would probably do this before applying anything to the skin.