Thanks for all the replies.
I ended up following the ER vet's recommendations, against my intuition. I just didn't want to panic and make things worse than they were, though I now wish I had taken him to the ER.
Despite my regret, Maxwell is acting now (as he did yesterday) as if nothing ever happened. He saw his regular vet today, who diagnosed him with a pinched nerve. She suspects he may have damaged one of his spinal discs as well, likely while doing regular Yorkie things like dive-jumping, bounding, pogo jumping, etc.
She performed some physical therapy and laser therapy in her office, prescribed a few more exercises, and will see him for a follow-up in two weeks. At that time, he will be given more exercises to help strengthen his core.
Thanks again, everyone. As a first-time toy Yorkie owner, I'm still figuring things out. Hopefully, this will be the last medical event for awhile.
