Could be mange or tics, and flea bites that are infecting him through biting. When this tics bite dogs, they give them infections that can be both harmful to dogs and to humans as well. Go to the vet, and have them scrape his skin to check for either of these mites that could eventually kill the dog or cause them severe discomfort.
Hot spots are caused by matted and/or thick fur which holds body heat and moisture close to the body, effectively rotting the skin underneath. The dog is shivering from pain and/or fever. The area must be shaved and medicated. The dog will require immediate veterinary care for wound treatment and necessary medicines! Please do not delay, your dog likely is in severe pain!