Sorry to hear this. This can be very frustrating for the dog and the owners. It could be from fleas even if there aren't any visibly present. (They are fast little critters! A flea can travel one mile per hour!) Also, the early stages of mange is a possibility. There are different types of mange. One of the most common mange is Sarcoptic Mange (aka "Scabies") which is contagious to humans and from animal to animal. They are microscopic bugs that actually dig tiny tunnels into the skin and cause very intense itching, little red bumps, inflammation and hair loss. The 2nd most common mange is Demodectic mange and is mostly discovered in younger dogs. It is a parasite which lives in hair and oil glands of the skin and multiplies very quickly. It is also a genetic disease so if this turns out to be the case, it is highly advised to avoid breeding any infected dog even after effectively cured. Demodectic (aka Demodex) doesn't cause much itching symptoms, but the affected areas are very susceptible to secondary bacterial infections especially if treatment is not given in a timely manner. Both types of mange I mentioned are treatable, but keep in mind that sometimes (and often) it takes patience and consistent treatment to finally rid the dog of the problems. Another reason could be a rash. Rashes can be caused by several different things, but most commonly they are bacterial in nature. Other rashes can be related to allergies, yeast or even fungal infection. Without being able to physically see your dog, it‘s difficult to say exactly what is causing this. Each type has a different treatment. Because of this, I recommend having him seen by a vet so they can accurately diagnose the problem.
Each type has a different treatment. Because of this, I recommend having him seen by a vet so they can accurately diagnose the problem. If you want to try something first, before seeing the vet, minor bacterial infections can be treated by washing the area using benzyol peroxide. You can try washing the area and make sure to thoroughly rinse the area. You can do this twice daily for a few days to see if it is improving. If it is a more severe rash or cause, the benzoyl peroxide will most likely not solve the problem. For the scratching, it wouldn't hurt to apply some Frontline Plus in case fleas are the problem.
Regardless, I do suggest a vet visit. If mange is suspected, he or she will do a "skin scrape" and look at the sample under the microscope. If not, they can properly diagnose the problem and prescribe the appropriate medication and treatment. Here is a link you may find helpful, and give you other things to try too. http://www.marvistavet.com/html/itch_relief.html
I hope these suggestions and possible reasons for your boy's troubles are helpful. If you need any further help, please feel free to let me know.