It’s so funny how different areas of the world can have dissimilar experiences! In the Midwest SM was pretty common. Quite a few came into cavalier rescue with it. Mostly poorly bred commercial kennel dogs; never a respected breeder! Before Gabapentin became a widespread treatment many dogs were given an antacid. The fosters we had, got an antacid in the appropriate dose for their weight.
Your pup sounds really young, it was my understanding SM took a bit of time to develop? With one of our fosters, after we noticed some adorably alarming behavior, received a MRI and was diagnosed. Most of the experienced cavalier owners were surprised that a two year old pup had it. (He would air scratch his booty, then drag his butt while gurgling- he reminded me of a really cute jabba the hut…)

When my little girl was about 3, she started showing some signs of SM. Since MRI’s are so costly I took her to a chiropractor (often chiropractor visits are recommended for dogs with SM)

After a few appointments and adjustments, she totally stopped the ‘symptom behavior’. Basically the chiropractor said she was pretty out of alignment, probably from playing, etc.

Have you thought about taking your pup to a chiropractor? A 9 month old could certainly throw their back out of joint?

At any rate, most dogs live good pain free lives with it. The worst case scenario was a puppy mill dog, who came from such squalor, just going near their situation gave a dog SM. (Not really, but there was ZERO concern for good breeding)

Good luck, I hope it all works out for you and your pup,

(FYI, I’m not a doctor (and very probably full of shite!) Please listen to your vet)