Hi Gerry, welcome to the forum, would you like to write abit more about your dog's aggression issues - like what situations your dog is aggressive, what happens - what your dog does, what you do, and post it in the "Behavioural Issues" forum. Thanks.
It is not my dog, but in my wider family there is a Shih Tsu that behaves aggressively when on the lead. He doesn't bite or anything, but he barks like a maniac and he seems worse with bigger dogs. I think it id based on fear, he is a rescue do so we don't know why this started. They have been working hard with him to overcome this, but it has been a slow process. In other ways he is a lovely dog, he does not have a sunny nature, he is the 'Eeyore' of the dog world, but he is friendly and well behaved. I have become very fond of him.