My name is ***** ***** I’ve been involved professionally with dogs in the health and behavioral fields for over 18 years. It will be my pleasure to work with you today.
This is a hard time for your boy. he has been uprooted from his home and moved to a new one. In addition, he has moved into a house where this is already a dog. Dogs are pack animals and when you put two dogs together, one has to be the boss. Usually it is an older stronger more dominant dog. In this case, your dog has been the top dog in your house and her dog has been the top dog in his house. now you have two dogs who are both used to being the alpha dog. Since you moved into his house, it is unlikely he will just let your dog take the top position thus he expects to have the privileges the top dog has.
Alpha dogs have privileges such as being fed first, going in and out of a room or house first, getting attention first, etc. You are used to bestowing these things on your dog because her was an only dog. Now however, you are in a multidog household and everyone will need to make adjustments. Since the younger stronger dog is the best choice of alpha, and the lab also already "owned" the territory, he is the best choice of alpha. Your dog won't like this so it is up to you to reinforce this. I know it will be hard for you, but you need to show the lab attention before your dog except for when you are in your own room. If you are giving treats, he gets them before your dog. That helps your dog see that you acknowledge the other dogs place as alpha and will help your dog make the transition.
The other part will require your dog to go through more formal obedience training. It doesn't have to be classes but it will need to be structured formal training by you. This means a couple of training sessions each day for a specified period of time. You will practice the same commands over and over again. The following site is helpful in helping owners train their dog. Be sure and click on the link to the page on obedience at the bottom. and links on subsequent pages leading to detailed instructions.
Training works best if you train at least 30 minutes a day (two 15 minute sessions). I would start making your dog work via the Nothing in life is free program (NILF). It is outlined below.
You will focus on sit and down and be sure to have a release command so he knows he has to stay until you say the release command. That is the number one command most people forget and then the dog learns that he decides when to stop sitting or laying down. So don't forget a release command. This will allow you to have him sit or lay down and NOT attack the lab. It also makes him realize that you are in charge and it is up to you who you pet or do not pet.
In addition, you need to keep a leash on him and prevent these attacks. I would recommend that both dogs be leashe. If one dog even looks at the other dog, a correction should be done. Any sign of aggression including a prolonged look, hair raised on the shoulders, a growl or even a stiff legged walk, should be corrected. A correction is a quick tug of the leash and a firm low toned "NO". Once you have done this couple of times, you should notice the dogs ignoring each other. When that happens, you will want to reward them for the desired behavior. Again, use tasty treats like the hot dog slices. This teaches the dogs that you WILL not tolerate fighting in YOUR pack. Your aim is to be the boss of both dogs.
It is going to take some time and work on your part but it does work. If these techniques do not work, you might get your dog checked for a thyroid issue. A thyroid problem can cause sudden aggression but I suspect it is just a dominance issue between the dogs that will work itself out given a little time.
I hope this information is helpful to you. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have . If you do find this helpful, please take this opportunity to rate my answer positively so I am compensated for my time.