HELP! My German Shepherd has getting very dog aggressive and I don't know how to correct it?

Alright, I have a six month old pure German Shepherd male, neutered. We've had him since he was about three months old. He was very shy, didn't have much socialization with dogs. We took him in Petsmart & Petco a lot and he quickly got accustomed to dogs. He showed no signs of aggression at all. Then all of... show more Alright, I have a six month old pure German Shepherd male, neutered. We've had him since he was about three months old. He was very shy, didn't have much socialization with dogs. We took him in Petsmart & Petco a lot and he quickly got accustomed to dogs. He showed no signs of aggression at all. Then all of a sudden, maybe a month ago he started barking at other dogs and small children. At first it was just a little bark and then we'd distracted him and he'd be fine. We thought he was just saying hello in doggie language or being playful. Slowly it progressed and now he is barking feriously at other dogs, like he wants to rip them to shreds. He's like that with small children too. Normal people he's fine to and is friendly etc.I just don't understand what triggered it and how we can fix it.
Update: Oh, and I forgot to say that we take him to Petsmart obedience classes he finished beginner with a Male Jug & Female German/Chex Shepherd Mix. No aggression at all, just playtime. He got slightly aggresive to OTHER dogs in petsmart toward the end of the classes (it was eight weeks old). We now enrolled him in... show more Oh, and I forgot to say that we take him to Petsmart obedience classes he finished beginner with a Male Jug & Female German/Chex Shepherd Mix. No aggression at all, just playtime. He got slightly aggresive to OTHER dogs in petsmart toward the end of the classes (it was eight weeks old). We now enrolled him in Intermediate Classes in Petsmart and there was a Male Pit Bull enrolled too. He barked and act so aggressive, we thought if the lease slipped off our hands he was going to kill him. It scared us so much. The trainer said it might be triggered either by two things. 1. Our outside dog who was aggressive to him when we first got him but now they seem to be buddies. Or 2. Our neighbors dog across the street who barks JUST like he does now. They have never met because our dog is always on a lease outside and the other dog is trained not to cross the street (Invisible fence?). But the trained didn't really say how to help the behavior other then use 'leave it'.
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