Unfortunately, as your puppy missed learning bite inhibition by being taken away from his mom and siblings too early. I suspect you let him get away with play biting so he's now moved on to the real thing.
I'd suggest trying the Nothing in Life is Free training program. You need to establish yourself as the boss without hitting him. (A smack in the face will only make him more aggressive).
Nothing in Life is Free Gaining control of your dog humanely Basic Training Techniques | The Humane Society of the United States
If this doesn't work, you need to consult a behaviorist. Biting is very serious and can lead to lawsuits or euthanasia. It should be taken very seriously.