I thought my ears were burning lol
G'day Ms Koala, and yep you're right, we have 3 BSH's, a blue f, a blue & white mitten m, and a silver point f.
BSH's take around 4 years to reach full maturity, but by nine months, your cat should pretty much look the way it's going to, but, as your vet has indicated, I'd take a wait and see approach, maybe you cat is just immature in its physical development. It's really hard to say without seeing what the cat looks like.
The main thing, is that the cat is happy and healthy, with a bit of luck, the growth rate of the head catches up with the rest of the body, during the first 4 years of growth. I can't really offer much more than that, but your cat is in a good home, so it's all good.
Here's a picture of our 3