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.
I totally understand your concern. It isn't going to be easy but it can certainly work out. Both dogs are going to nee obedience training. It would be best if that was started a few weeks before you introduce the dogs to one another. 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.
This obedience training will allow you and your son control over the dogs which will help prevent fighting. It also establishes the owners of the dogs as the bosses.
To have the best chance of the dogs getting along, you should introduce them away from your house. If possible make it an area where neither dog has been before so that neitherdog feels it is their territory. On neutral ground they have a chance to get to know each other without the need to protect their property. Both you and your son will need to practice not tensing up while introducing the dogs. If the dogs feel you are tense and worried, they might think it is the other dog making you feel that way and become aggressive to protect the owners.
Let the dogs sniff each other but don't just let them loose with one another. You should both learn body language so you can determine if either dog is acting aggressive or dominant and correct the behavior with a little tug and firm NO. Read about this here:
If possible try to introduce them to each other a few times before bringing Dizz into Mia's area. With Mia being an older dog and hopefully intact and Dizz being fixed and younger, they may not have a problem since Mia is the logical choice of alpha between the two dogs since she is older and it is her territory. If Dizz submits to Mia, then there shouldn't be a lot of problems. They may have a few issues during the adjustment period but as long as there isn't a lot of bloodshed, try to let them work on it by themselves. Now if Dizz is male, then there is a better chance of them getting along since the most problems occur between same sex dogs.
Once living together, additional training can also help. Both dogs should be leashed and 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.
The Bat method may be helpful as well once the dogs are together.
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.