Blood tests came back normal, and nothing visible on the x-rays.
Vet says he going to be treating the symptoms, so he's assuming it's toxic reaction and/or a back problem, so that's what's being treated. Vet said that Spitz isn't looking any worse, and possibly walking better, so that's good at any rate.
I'm going to ring back between 5 and 6 for an update. Current plan is for me to grab Spitz and take him home for the night (he'll be more relaxed at home in a familiar environment) and then possibly take him back to the vet tomorrow morning, depending on what they recommend.