First, throw all the talk about dominance out the window. If a dog is truly dominant, getting in a confrontation would end badly for the human, even with a 9 month old pup.
In all likelyhood, it's simply an ill-mannered untrained pup. What training does he have and what mental exercise does he get every day? What exercise does he get daily? What is his schedule like? What are your expectations for his behavior that you've enforced since he was a puppy? Is he an indoor dog or an outdoor dog?
Definitely get a trainer or behaviorist involved. Talk to people that you know with well-behaved pets and find out who they went to. Start at least a basic obedience class sooner than later.
I agree. You may want to consider an e-collar for when he bites, but I'm not sure on that because I have never used one myself.If he does this only when you turn your back, don't turn your back. He is more than likely testing his boundaries and is seeing if he can be ALPHA.
Like others have said consult someone appropriate.
I see in your avatar the dog has a pinch collar on and is tied to a chain. A pinch collar IS NOT a "tie out" collar and is used in handler training. (not a great method). The issue leaving it on a tethered dog without a handler is the collar will make corrections at the incorrect moments causing serious confusion to the dog.
I agree. You may want to consider an e-collar for when he bites, but I'm not sure on that because I have never used one myself.
Absolutely do not use an electric collar on a dog exhibiting reactivity. This is never an appropriate training method for a reactive dog.
Please consult a trainer. The money spent on a good trainer can prevent a lot of psychological damage.