Have a professional come to the house to help. Sometimes dogs and cats don’t get along because they are unfamiliar with what the other one is but occasionally the dog can see a cat as a prey animal. In this case, your cats are making the best decision by staying upstairs. If we force the interaction we may be putting your cats in danger.
If they do come down, make sure there is no chasing. If the dog chases the cats, even just once in a while then the cats will always be afraid.
Another tip is to swap scents. Cats and dogs get to know each other through scent. Have the dog sleep on a blanket, the cats sleep on another blanket then swap. This way the cats will get to smell the dog and vice versa.
If possible, have the dog and cats eat near each other. Initially this may even be through a door. The dog eats on one side and the cats eat on the other.
Allow safe exploration of each other’s space. Crate the dog or take him on a walk and allow the dogs to come down an explore. Then move the cats to a secure room and allow the dog to explore the space. Repeat this for at least two weeks.
Next take your dog on an extra-long walk so they are well exercised. Put one of the cats in a smaller room where the cat can’t hide or run but has a safe place to perch out of the dog’ reach. A master bathroom or laundry room works well. Put the cat on the counter and give the kitty an extra special treat the cat will only get during this exercise. Next bring the dog in on leash. The dog should be given special treats as well and praised as long as they are calm. If your dog starts to act up, walk them out of the room. Wait 20 seconds and walk back in. If at any point either the dog or cat gets too stressed, stop immediately.
If you have more than one cat, make sure the dog gets exposure to both cats but only one at a time to start.
As the dog and cat spend more time together, then they should be able to stay calm in each other’s presence in the small area. Once they can be calm for at least 30 minutes, then you can start to practice in larger areas.
Once the dog can be calm with each cat in larger areas, then you can introduce any other cats you may have into this area.
Remember: Never force any cat or the dog into a stressful or dangerous environment.