Your dog may have flea dermatitis , this does not mean he has to have many fleas, it only takes one to hop on, bite and hop off to cause a reaction. Also Goldens are prone to hot spots usually caused by allergies or flea dermatitis. If the spot on the tail is red and inflamed and the size of a quarter (or dime) he has probably developed a hot spot from constant chewing. These can be painful and may require antibiotics if infected. I am going to direct you to sites regarding hot spots and flea dermatitis.