Had my boy for six weeks - got him at ten weeks. On arrival he was very, very bitey and would bark and growl back at me if I spoke in a firm voice and told him "no!" Over six weeks he is loads better, less nipping and responds to being told "no" and obeys well. He's actually a great little chap and very food motivated so training is going relatively smoothly, his recall is excellent. However he has now on four occasions been very nasty with other dogs that we've come across and I mean properly going for them, even much bigger dogs than him. A friend said I should let him off lead to socialise and let those dogs take him down a peg or two, but what if he really hurts one of them? it just sounds the totally wrong thing to do. I want to nip this in the bud before it becomes ingrained. He's booked in for puppy obedience but those classes don't start till January. Any ideas on how to handle this in the meantime? He really is a lovely boy in every other way and is devoted to us and has improved in leaps and bounds since we got him.
I don't have direct experience with terriers but as a general rule I think you have to persevere with socialisation rather trying to discipline that aggro response away, as he might interpret the discipline as confirmation that Other dogs = Situation.
Are you sure he's going for them aggressively as opposed to an exaggerated play behaviour? Is it fear aggression or maybe dominance? A good behaviourist might help you read what's going on. If it's dominance often the best teacher of manners is other dogs, so as far as possible let them sort it out.
If it's fear based, it's a question of desensitising him to strange dogs by meeting as many as you can in a calm controlled situation. How about spending part of your walk just sitting on a park bench with him on the lead? Let passing dogs come and go. If they come to say hello, and he does well, you can praise and reward. Greet other dogs yourself to demonstrate they are to be viewed as friendly. Good luck