Cat with broken or dislocated leg?

Our cat is about 13 years old. We noticed just over a week ago that his back legs were moving strangely. He was struggling to move them. Now it has got much worse and it appear to be affecting just the one leg. The problem coincides with him having his front paw raised a lot so we expect some kind of accident. He is now dragging his back leg and when he lies down the leg is not lying flat it is horizontal off the ground. Do you think it is broken or dislocated? Thanks

The cat needs to see a vet tonight.