Hi XXXXXXXXXXXX~
You must get your dog under control immediately before he either does serious damage to someone or someone's pet or is euthanized for being a dangerous dog.
It sounds to me as if you have a fearful/unsocialized dog who gets aggressive when he is faced with new situations that he doesn't know how to deal with. Unfortunately, this is one of the toughest behavioral problems to work out, but with a great deal of patience and understanding, and most importantly consistency, you can get her to break these bad habits.
Labs are loving, playful, fun dogs. They are lively and courageous. They are very strong willed, and can be stubborn making training them a challenge from time to time. They are extremely loyal to their owners and that is probably part of what you are seeing here. . Your dog is still relatively young, and I feel he needs a lot more socialization or you may end up with a dog that is a problem to visitors and may end up a problem for you if he decides to take his 'back talking' to another level and actually bites someone. While a quick fix isn't available, until you can resocialize him, you can let your boy trail a leash and give him a quick correction (a tug at the leash quickly and hard) and firm "NO" if he shows aggression toward any person who may or may not be on your property.
They are quick learners so hopefully he'll learn quickly that this behavior is not acceptable. I also think you will need to enroll him in an obedience class not only for training but also for the socialization that occurs at classes. It will also establish you as the alpha dog in your 'pack' at home, and a sub-adult dog would never even dream of trying to bite the alpha.
I hope this helps.
Certified Veterinary Technician
Associates Degree in Veterinary Technology. State and Nationally certified.