Hi this is DRB and I understand your situation very well as I have dealt with this so many many times over the years. You can seek behavioral therapy for this if you choose to but it is very likely that the best you will be bale to achieve will be managed situation with keeping them apart. Dozer does not have to be an aggressive dog or bad with people to do this type of behavior.
One thing I try to explain to all clients is that the nicest dog in the world can do this type of behavior. Dog's by their very nature are predators and living with them we unfortunately forget this fact. But some breeds like Pitts have their predator instinct just a little more at the surface than say a Shih Tzu like your other dog.
Dozer does not realize that he is doing anything wrong but on that first attack he would have killed your other dog if you had not been there to stop the situation. He sees your other dog as prey for whatever reason......there is nothing you can do to change that perception.
Pitts can be absolutely wonderful dogs but the majority of them cannot live in harmony with other animals, neither cats nor other dogs especially small dogs because they will ultimately perceived as prey. Most Pitts do better in a single household as the only dog. I hope that makes sense.
If you are determined to try to make this work then I can help you find a professional behaviorist but I can tell you that they are going to give you close to the same advice that I have given to you. But I am glad to help you locate someone.
Ultimately for your Shih Tzu the very best thing you can do is find Dozer another home where he is the only dog preferably with someone that really knows dogs well. I have some very wonderful Pitt patients in my hospital but every one of them would attack another dog if given the chance. I know this is not what you wanted to hear but this is what I have seen in 26 years of practice. I do hope this will help you and feel free to ask any follow up questions as I will be happy to help you. I check my computer every night but it will be late tonight as I have a very long day today.Sincerely, DRB
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