21 months old is not the age he cannot be retrained. He doesn't trust people, that is the fact. Do not invite any kind of visitors, as those who do not know dogs, probably, have approached him at the front and raised a hand over his head - that is a sign of human agressiveness in a doggy language. Strangers and presence of other dogs increase his frustration, and his self-protectiveness with bites is quite understandable.
First of all - don't try to exercise your own dominance over him, let him to relax. Speak in a low gentle voice and avoid high pithed tunes. It simply takes time for him to come back to his senses, and don't be so quick to call him "fear agressive" because, most likely, he expects any moment to receive pain instead of treats every time he sees a human. If there are any visitors - ask them to take a sit, let him to approach them only if he wants, and feed him something tasty, not to pet him. It could have been nothing but this "petting" that triggered the trouble. Not every human realises, that some dogs are scared of being petted.
You should look for resque groups who would agree to take him on, he has good chances, because he is young. Meanwhile, I'd suggest you to muzzle him in a soft adjustable leather muzzle, thus providing maximum freedom for both, yourself and him on your walks. If he is not trained to wear a muzzle, put it on just before going out, and take it off when you are out. He should allow keeping it on for longer first, and all the time in couple of weeks.
Last edited by David Taggart; 02-23-2014 at 06:40 PM.