Hopeful for a change

Thank you everyone for your advice. We returned the shock collar and have switched trainers. We start next week, so hopefully we will have more positive results. Thanks for the book suggestion as well, I will look into it. I am a first time dog owner, so all of this advice is great.

My husband and I adopted a BC 5 months ago. He is a male 2 year old. When we first got him, he was perfect. He was introduced to my mother-in-Law’s dog (cockapoo) the second day we had him and he has always been fine with him. He also seems to be fine with the neighbors dogs. After a month of easy walks and near perfect behavior our dog started becoming aggressive towards other dogs. He lunges and barks at other dogs and can be challenging to control at times. We tried avoiding other dogs. We tried distracting and positive reinforcement (not effectively). We switched to a no-pull vest,