He's still a puppy, even though he looks like an adult. If he keeps nibbing at one particular area, check if he suffers flea problem or other skin problems. Golden retrievers are genetically pre-disposed to many skin problems, so you should be extra careful with this.
It's also possible that he's simply bored. Do you leave the dog alone during the day? How many hours do you spend with your dog and how much exercise do you give to him?
Oh he's a beauty! What's his name?