The obvious guess would be "ring" worm... But it could well be a wheal due to a bite or even allergic reaction, although they are usually raised and itchy, and you didn't mention itchy. Lyme's disease is also associated with a ring, but my understanding is that it's ring is much larger. Ringworm doesn't itch, and has nothing to do with worms. It's a fungal infection that is in fact, transferable to humans (mostly young children) and other pets, so you'll want to keep him off of common area surfaces as much as possible, and wash his bedding often to keep down the number of fungal spores in the environment. Ringworm works its way outwards from the initial point of contact hence the circular zone looks to be expanding (the innermost area of the lesion is the area already healing and the outermost raised area is the area where the ring worm is the most active) Microsporum canis, the most common form of ringworm found in dogs, lesions infected with this particular type of ring worm will 'glow' under yellow-green fluorescence when you shine a UV lamp on it. The good news is that ringworm generally runs its own course in a matter of 6-8 weeks and the symptoms go away own with or without treatment, but having said that, most vets opt to treat ringworm in the animals that have it to stop further spread to owners or other animals. There are several means of treatment, and your vet can help you decide which one is best for your situation.
