He's already doing much better. He was completely holding his left rear leg off the ground almost all the time a few days ago. We thought maybe he'd gotten something in his paw and got infected, that's happened before, but he wouldn't really let us look at it. Next day he went to the vet, definitely not in the paw, most probably some kind of ligament thing. (I also am told he was very good and didn't even growl.)

They gave us a prescription for anti-inflammatory. From looking at him now, you wouldn't even guess he was ever hurt, but they also told us that it can take months for such an injury to heal, and you really want to make sure it's good and healed so he doesn't put all his weight on the other leg and hurt that one, too. So we'll give it at least a few weeks before we run him around again, just to be sure.