We have an itallian greyhound jack russel terrier and he is only 5 years old. He will not willingly go up and down stairs, he won't stand up with his paws on me for me to pet him. (which he usually does ALL the time) he won't jump up for a rope or a treat. And he won't stand for very long. He will either sit or just lay down. After laying for a while his back legs will start shaking slowly and then after a while they will start shaking like he is freezing. I've felt for tense muscles and can't feel any. He seems sensitive to me touching his hips and butt and back legs. What is wrong? And what should we do?

He's in pain, from what, who knows (often it's from being bumped by a car, is that possible)? He needs X rays now!

I agree with Joy, if he is reluctant to do these things that he usually has no problem with it seems that he is experiencing some pain and discomfort. Back or hips would be a guess, but only a guess, best to get to the vet and have him checked out.
As already stated you should likely be prepared to have X rays taken.
Good luck