Disorders of the Corpus Callosum is a loss of white matter, but it is not a progressive loss, basically the corpus callosum did not grow or grew inportioins or too thin. Nate has Hypoplasia of the Corpus Callosum and is considered moderately affected. There is a huge spectrum of outcomes for callosal disorders.
I have a friend with two sons who have a non-progressive form of leukodystrophy. Basically what they have for myelin is what they have. It won't get any worse or any better. One boy is more affected than the other. They both also have hearing loss. It is considered genetic, but they have not found the genetic change for their issues.
You really have to wait and see what you are working with. Worrying and googling before you have a diagnosis will take up a bunch of energy.
You could try and get the radiologists report and read that. I would contact medical records. What your dev pedi told you seems really vague and not helpful at all. Perhaps they were vague because they didn't want you googling and making yourself worry more than necessary.
The brain is such an awesome organ and can be affected by genetics, environment and either good luck or bad. It's a "wait and see" life and a true lesson in patience. Hugs.