Was he exhibiting this behavior at the time you took him to the vet? If so, did the vet do specific testing to rule out medical issues that may cause aggression - such as thyroid?
At this point, please do not even consider breeding your dog. He could have either health or temperament issues that should not be passed down to puppies.
Natty Boh 6-27-12
Annie 1998 - 8-2-12 RIP