Aggression toward humans is like cancer, once you've got it you'd better take fast action or it will get worse. The causes and cures for aggression are many but because of the inherent danger to you and to your dog, please seek qualified professional help immediately. If your dog is growling, snarling or snapping don't wait for a bite, deal with it now. Neutering male dogs is not optional. The vast majority of serious dog attacks are by intact male dogs. NEUTER!
Regardless of the root cause of the aggression, it can only be managed by you changing what you do. Note here we say “managed” not “cured”. In order for a dog to change what they do, you MUST change what you do – first! Correct the relationship and the aggression will be easier to deal with.
Once the relationship is straightened out, the aggression can go into remission – but it is not gone. It is very rare that aggression is ever “cured” rather it is managed. Managed by you.
Avoid the pitfall of rationalizing your dog’s behavior:
"He doesn't like..."
"He was just scared..."
"You shouldn't have... "
No.
The reality is your dog probably had a choice. He could have walked away, he could have shut down, he could have done something else – he chose to threaten a human being. This is always a serious problem, even when your dog is wonderful 98% of the time. It is easy to be a good guy when life goes your way. The test of an aggressive dog is not what he does when all goes well but how he handles frustration and stress. It's that 2% factor that is dangerous!