What's on my dogs back?

The vet doesn't know what this is. I've tried antibacterial spray that they gave me and oral antibiotics. If I touch it or he accidently rubs something against it his back leg starts involuntarily moving like he's trying to scratch. 

We call these 'hot spots' but that's no diagnosis; it's a description and this one sounds to be very itchy and perhaps a bit greasy. Sometimes they can be caused by pain on a deeper level, the hot spot secondary to itching.  There's also a place near the tail where a lot of grease is produced which can be targeted by superficial infections.  I wonder whether they have considered ringworm? - depending on the location and the circumstances. It can be frustrating when vets say they don't know; rather than angling for a single answer, maybe ask what the main differentials / possibilities are.  Theirs might be different from mine.   From there they will try to narrow it down, often using trial treatments and tests. Please do let me know how you get on