Jeff,
The same thing happened to Psi when he was about 3 months. He was fine one day and the next he couldn't move his back legs at all. It turns out he got the "kitty flu", calicivirus. It was an upper respiratory "cold" that spread and got into his legs. He had a fever of 105. We took him to the vet and they hydrated him and gave him antibiotics to fight any stray infection, just in case. We had to feed him every 4 hours or so but he got over it in a few days. So, if you cannot get to the vet right away, check to see if he is dehydrated by pulling up the skin between his ears (if it snaps back down he is hydrated, if it takes longer than a second, he is DE hydrated). Also check his temperature. Then, get him to the vet as soon as you can so they can hydrate him and get his fever down.
Good luck!
tanyuh