There are several possibilities for what you describe This could be an allergic reaction, bacterial infection, fungal infection, or autoimmune condition. It is also possible that you could be looking at a certain type of cancer in dogs called mast cell tumors. These can occur in multiple locations, but usually don't grow as quickly as you describe. However, because they are serious I wanted to mention them as well.

Unfortunately, many skin diseases can look very similar and often will require some tests to help determine what is going on. It is likely that your vet will want to do a skin scraping , impression smear, or needle biopsy so they can be an idea of what is going on. For some skin conditions, a full biopsy is necessary to find out exactly what the diagnosis is. Luckily, all of the possibilies I mentioned above do have medication that can be helpful - most can be cured, although some of the autoimmune diseases can be managed but not really cured. Treatment for mast cell tumors involves removing the growths - additional treatment may be done depending on what grade of tumor is present.

Because the problem seems to be spreading and is painful, I would recommend getting him checked out by your vet as soon as possible. It is likely that this will turn out to be very fixable, but it is important to find a correct diagnosis so the right treatment can be started.

I hope this answers your question about what could be the cause of the strange lesions you are seeing on your dog. Please let me know if you need additional information.