I have a few thoughts on this, as a very dedicated dog mom and as someone who has come out of an abusive relationship.

First: It’s phenomenal, in all the best ways, that you recognize you have been irresponsible with your dog and his lack of training. You definitely made a big mistake trying to introduce an untrained dog (predator) to a chicken (FOOD!!). Dogs need to be trained very slowly and carefully, over time, and tested multiple times in safe situations before being allowed to enter a situation that could result in harm to either them or anyone/thing else. It was way too soon for the chicken test, but, you have recognized that mistake and you are ABSOLUTELY doing the right thing by consulting a professional trainer. 🙂 You are NOT wrong to keep your dog! You haven’t been a great owner for him. You haven’t taught him how to behave in certain situations, and you have put him in those situations anyway and HE is the one who has suffered by being hit just for behaving how a normal dog would have behaved. None of this is the dog’s fault persay- yes, he did bite, but he was scared and the situation could have been prevented by better judgment calls from the human in his life. You are fully aware of this, you are taking responsibility, and you are acting in his best interest now. You are making a great effort to become an awesome dog mom to him and I truly believe that if you commit to working with the behavioralist, you can turn it around and have an amazing life together. 🙂 I know some of that will feel harsh to read, but, it’s simply a statement of the truth you already acknowledge inside yourself. One that will soon be untrue from the sound of it! 🙂 Your dog isn’t biting because he’s a bad dog- he is a fearful dog reacting in a way natural to dogs who have a stronger fight than flight instinct. This can be remedied!

Second: I will say this once, and I will say it very clearly. Any human who will harm an animal without questioning, especially one who will encourage others to harm animals, is not someone you should trust. The fact that he thinks hitting a dog is some kind of training is outrageous. Negative reinforcement is probably the least successful method anyone could want, and the most damaging- really, it’s just mistreatment with a flimsy justification behind it. Most people who abuse their children and spouses have these tendencies, and most serial killers start out by torturing animals. There’s a difference between ignorance-driven violence and deliberate violence- your situation is probably ignorance-driven (I hope), as your boyfriend probably just doesn’t know much about properly training dogs or working with rescue/abuse dogs, but the motivation does NOT excuse the outcome of the action. You should have a serious, strong discussion with him about hitting your dog and your own feelings on it, and you should absolutely speak to the behaviorist about what your boyfriend did (hitting) and how your dog reacted (biting then fleeing). I recommend NOT involving your boyfriend in the sessions with the behaviorist, because you already know he will bring only negativity and could prevent it from being a successful lesson for your dog.