I have a BTS who has a nose whistle too, my vet says that it is from a build-up of keratin in the nostril. It generally takes care of itself over time, and is nothing to worry about. Just keep an ear on it, make sure it doesn't seem like he's having a hard time breathing.
I'm no expert, but 10 1/2 inches at 3 1/2 months old sounds normal to me

Breeder and keeper of Meraukes from 2010 to 2022.