Welcome to Yorkie Talk, and congratulations on your new baby Charlie.
He is pretty small, as you might have expected coming from such small parents. Growth charts often say as a rule of thumb, their adult weight will be double the weight at 12 weeks. But, it can vary.
There is a lot of new puppy info here that might be helpful to you:
http://www.yorkietalk.com/forums/gen...w-parents.html