American Bulldog aggression

Question

Buddy and Rez   I have a 2 year old American Bulldog and a 1 year old mix dog who is half American Bulldog and half Retriever (we think). When we brought the mix dog home as a puppy (his name is Reznov), Buddy (theAB) took to him right away and they have always been the best of friends. Everywhere Buddy would go, so would Reznov. Everything Buddy did, so did Reznov. I have been fostering some dogs from the shelter lately. Usually it was females but recently I had a male Pitbull puppy named Pirate Jack. Pirate Jack was blind in one eye and very thin, obviously he had been in a bad place before being in the shelter. But he got along really well here except for food aggression so I always made sure to feed him separately from the other two. My two dogs always ate out of the same bowl together with no problem. Anyway, the foster dog and my dogs all seemed to get along really well. Now and then I would catch the foster dog chewing on Reznov's ears a bit too much but Resnov didn't seem to mind and would kind of play right back at him.
My problem is that yesterday Jack went to his permanent home, and now Buddy is has started being aggressive toward Resnov. It is like someone just turned on a switch inside him. One day he is good, the next he dives into Resnov for no apparent reason. Resnov now has to slink around the house trying to stay out of Buddy's way where as before he could always jump on him and play without fear. Why is Buddy suddenly so mean with Resnov? They can both be just laying on the floor then Buddy might get up and I see him looking at Resnov with a stern, intent look and I know he is about to start up again. Resnov may even not be looking and aware he is coming. What is making Buddy act this way?
Any insight into this would be appreciated. I love Buddy. He is my darling and I don't want to let either dog go. Resnov is the sweetest dog in the world and I can't stand the thought of him getting hurt. What can I do to resolve this?

Answer
Wow, it sounds like having the foster leave made a real impression on Buddy and therefore he is making a new impression on Resnov!

What I think would be good is basically starting from scratch on an introduction between the two. That means feeding separately as well as giving attention separately. What I want to make sure is that there is no aggression towards you when you give Resnov any attention.

What I want you to do is a process called "earn and reward." This both reinforces that you are the upmost leader in the household and will allow Buddy to remember that Resnov is a member of the family as well. Basically with everything rewarding that is done, including feeding, treats, walks, etc. They, especially Buddy, is to perform a command and does not get the reward until the command is met. An addition to this is allow Buddy to be the second in line to get anything. For some reason he is acting out and simply needs to get reminded that he is not the almighty leader of the household.

I also want you to pay close attention and watch for signs that Buddy is going to react. Don't hesitate to give a no command before he even reacts and reinforce it with stay or similar. Then reward when he is behaving appropriately. You can also place a leash on Buddy to give you added control when necessary to give a slight tug for a reminder that no means no. You would obviously hold the leash or place is around your foot. Just so that it's easy within your grasp when you need it.