Thanks for answering. Yes I also believe medical conditions play a part in temperament changes to a dog. I'm hoping his short episode of fear and aggression is something he can get over and go back to being a happy, confident, dominant boy. Your 'trust' comments were interesting, when I think about it, he really does need to learn to trust himself again. I also have to admit, I need to trust him again. Easier said than done, however I have been exposing him to various dogs (in dog parks - hate them but they have their uses) maybe about once a week and I muzzle him (this is for my comfort). First time he has ever been muzzled, but he accepts it as though it is not there and it really has been helping both of us to trust again. He seems much more calmer and is learning to greet other dogs properly.