Hi there. I don’t want to alarm you at all, but something very similar happened to my baby. She’s still with me, but she had sudden partial paralysis and she was in the emergency vet for three days.
She was running and then suddenly her back end wasn’t moving properly or at all and she was howling. I rushed her in and they kept her, after those first few days they decided it was a blood clot in her spine by the base of her tail.
It was a long road and we’re still on that road two years later, but it was touch and go. I would mention a spinal blood clot to your vet to get their take/see what they can find out as these seem like the most similar cases I’ve come across—or at least you’re the first person I’ve heard that has a simile story.
Thankfully your little one doesn’t seem to have it as bad, because Liz Lemon was, I’m pretty sure, 50% paralyzed immediately.
Please take care of your buddy, as I’m sure you are of course. It was a very scary situation and has definitely changed her life (and mine, as we are dealing with partial incontinence), but she’s managed to continue to live a happy special kitty life. The burden is now mostly on me, which I’m happy to take on so she can thrive.
Let me know if you have any questions and best of luck to both of you!!
Edit: just as reassurance, mine was an emergency-vet-right-the-fuck-now situation, which thankfully yours does not sound like it’s to that degree. Her tail and back legs were affected, both inert, back legs more stiff while her tail drooped. Again, you seem to have a better situation on your hands, relatively, but the symptoms sound similar enough to mention to your vet. I hope this can help at least give you all one more possible cause to look at. Best to you all, I’m sure your boy will come through this just fine!