Here's some tips on walking a dog or training it to walk on leash.
Tyler Muto - YouTube
When you practice some of these techniques you can will have more of a chance to control the dog with the cats.
It is not so wrong how your bf's parents are keeping the dog. It gets food and shelter and is not chained up. But I believe a dog needs daily walks and clear leadership and enjoys living indoors with it's people.
Some people have just have strange attitudes to dogs.
I would take my time with a dog like that and introduce things to it slowly so as not to over whelm it. You can easily carry issues into the new environment like barking when left alone or separation anxiety.
Basically research dog behavior as much as possible. Try to instill calmness into the dog when you greet it. And then he will start to associate you with calmness.
I really liked Cesar's Way by Cesar Millan. It is a good book for people with not a lot of experience with dogs. Watch any of his shows and see how he walks in and greets dogs. He presents calm energy not sound or eye contact or touch.
Most people who don't know dogs so good will go to a dog with a lot of movement and be like 'hey boy' and rub it and hug it when it jumps on them. This is totally feeding and nurturing excitement. This excitement makes it harder for the person to control the dog. A good dog trainer uses excitement to good use but knows what they are doing. I recommend first establishing boundaries and some basic obedience and then after as the dog settles introduce more exciting training.
First, master the walk and show the dog you are stable and in control.