I just did some reading (very little but some) and why are they sure this is what Colby has? It is hereditary and can be noticed at birth or within 2 weeks. It says the condition doesn't get worse with age. Also, that it is more common in several breeds (none were Yorkies).
This is where I was reading so just curious why the vet thinks it's Cerebellar hypoplasia?
Cerebellar hypoplasia in dogs