I completely agree in regards to take him out and around other dogs as much as possible in a controlled enviroment. I had a 10 lb terrier mix that was attacked by 2 akita's, a large vet bill later he phyiscally recovered, but mentally not as fast. Before the attack he was outgoing, loved other dogs, people, everything. He had absolutely no fear. After, he wouldn't go near a large dog no matter what breed, he'd usually try to hide and trembled. I tried to take him out around other dogs, and that helped, but it wasn't till about 2 years later when we adopted a large mixed breed that he started to come around. By the time he passed about 8 years later, we had a total of 3 large dogs and he'd been around countless others without any problems or fears. So I think it will just take time and lots of exposure to other large dogs.

In regards to the growling, I confess I'm not an animal expert so with that in mind, I'd pull him back, make him sit and tell him no. On the other side I'd praise him a lot if he goes near another dog without growling. Even when passing another dog on a walk, if their close and he doesn't show any aggression I'd definately praise him. I agree dogs, can pick up on your emotions, so try not to tense, just be aware of body language so you can avert any problems.