My first guess would be ringworm. Seems like theres a lot of ringworm going around lately for some reason. Ringworm is often reddish to purpleish in color and can be a little scabby. It is contagious to humans.

Mange would be my second guess (which is also contagious to humans if it is the sarcoptic mange, it is known as scabies in humans). Mange *usually* starts around the ears, eyes, mouth & feet not in the middle of the back. But that doesn't mean that he doesn't have mange, its just more unlikely that that is what it is.

BUT seeing as none of your other dogs nor you is affected it makes me believe that it is probably something else. Demodectic mange is possible as is a staph infection, neither is contagious.

Your best bet is to take a trip to the vets for a simple & inexpensive skin scraping.

Good luck!