This one seemed to explain things pretty well http://www.thepetcenter.com/gen/lyme.html

I would have thought he'd show a bit of stiffness or limping before total disuse though, but it always goes through my mind when there's a sudden problem with a leg after having seen it. There should be increasing signs of it if that is it though. But it wouldn't hurt to ask the vet if you feel any suspicion of it.