Thanks, Eric! He seems to be doing really well. He's walking normally, can get up and down stairs, and even wants to jump onto the bed and couch (which I'm trying to discourage). We've been doing short walks only, and I can tell that he's getting antsy. He doesn't understand why I won't let him gallop around the house with his stuffed gorilla.

At any rate, this is quite mysterious. He's still not his normal, bouncy self, but that could be either the pain meds or residual discomfort. I'm going to call the vet tomorrow and get advice about how/when to reduce his medication.
It's all very strange. I just hope it doesn't happen again.