If you can, I would take your cat to the vet's as soon as possible, as he possibly could have heart disease, and have thrown a blood clot. They sometimes have lameness from this, as I just recently experienced in my gray tabby a few weeks ago.
I took him to the vet's and he is diagnosed with a heart murmur stage 4 out of 6, and is now on half of a baby aspirin every 72 hours.
He started limping for a couple days, and then could not jump. I took him to the vet's on Monday and had blood work and x-rays done. The vet feels he threw a blood clot, and he wants me to have an ultrasound done, if possible, to see how badly his heart is affected by what is called "hypertrophic cardiomyopathy".
Your cat could have thrown a clot, so I would take him to get x-rayed and checked as soon as you can. It is very painful, if that could be the problem. I would not wait.
By the way, is his back leg cold to the touch? That can sometimes signal that he is not getting enough blood flow to the leg.
God bless and good luck!