I had rescued an 8 mo. old mastiff/boxer mix about 2 1/2 months ago. He was crated for most of his life; had scars under his chin and on his tail from being in a too-small crate. He had at some point broken his leg and had a limp. When we took him home he was very timid, sweet, and immediately chose me (a female) as pack leader. My husband and I began working with him. We have another 4y/o boxer lab mix who is incredible, affectionate and very understanding with the new puppy.
As our mastiff mix has grown more comfortable and sure of himself, he has gotten into a HORRIFYING dog fight with an unfixed male (not my dog), and later that night attacked my boxer/lab in the car on the way home. Things seemed to calm down after the scuffle. It has been a week since that dog fight, and again, the puppy attacked my 4 y/o (the were playing for about 10 minutes prior, neither showing signs of aggression). No blood was drawn this time, but I am at a loss.
He is loyal to me and me alone, and I get the feeling he would rather have me to himself all the time. Do I have any hope? He is doing well with general behavioral training, is good on a leash, etc. But I love and am devoted to my first dog and don't want him to have to deal with this threat of aggression all the time. Please respond with any advice.