It does sound like intense fear aggression to me... it can be very serious (and can easily result in a bite), and DEFINITELY needs attention from a pro behaviorist. He must be a very smart pup, but he needs more than group classes now. Any advice we give you, or other people who dont know you, is just generalized... a good behaviorist can evaluate your dog and give you specific directions and details. I went to a behaviorist for some fear issues with my Border Collie, and it was soooo worth every penny! Follow-up Obedience classes could not hurt.

Until you do see a behaviorist, make sure your pup is under control and supervision always! For now, try to start hand-feeding him his kibble (if he allows that), or feed him inside of his crate so that he cant bite anyone. You should start Nothing in Life is Free training... make him earn everything he gets (walks, going through doors, food) by sitting or laying down. Also make SURE he is securely inside of your house or your yard is locked when you're gone. You do not want him to get out and get into a bad situation (neighbors chasing him, kids running up to him and scaring him, etc) and end up biting some one. ACD's (Australian Cattle Dogs, KYS ) amd GSD's have a predisposition to be somewhat wary with strangers, but it does sound like theres a lot of issues that have nothing to do with his breed.. poor pup sounds like he didn't have a good start in life at all! Its awesome that you're his owner now, and you care enough to nip this in the bud.