You need to take your Dog to the Vet immediately. I would recommend a holistic vet who will not only address the symptons but solve the underlying problem.
Something you can do at home to help is add a good quality Cod Liver Oil to their food. It helps the skin and the coat while delivering Vitamins A, D, and E as well as essential Fatty Acids. My Dog loves it! You can purchase it at any good pet store or health food store. Also, lots of outdoor play and exercise. A good romp in the park or a swim will keep your dogs mind off the problem and get the blood flowing. Sunshine and lots of socializing is will help heal and prevent problems down the road.
But take your dog to the vet for diagnosis as infection may set in and can spread.
Kwinston is right, but can i add that some of the hairloss things are extremely infectious and contagious and until you know what it it, keep your dog away from others ansd exercise good hygiene yourself
alot of outside dogs get whats called mange, its a parasite that causes the loss of all body hair on the animal. Ive used a home remedy for mange many years and it works every time, drain your car oil and save it. cover the dog with the old oil and let it dry (keep it outside afterwards till the oil is gone or if its an inside dog find a way to keep it out for a day and then wash). it will not hurt the animal. once the mange is gone the dog will show more affection to you then ever before. If not the mange there is alot of smaller animals that have nerve problems that sometimes cause the animal to continuously scratch even after hair is gone. could be multiple things. just have to use judgement and see what kind of animals they have been around.