Welcome to Yorkie Talk and congratulations on your boy!!
Size is always a touchy issue...sometimes you can estimate but nothing is ever exact. A lot depends on his body structure/type. A short stout 5lb dog looks very different from a tall lanky 5lb dog.
Regardless of how much he weights he will bring you so much joy over his life. Enjoy the sweet innocence of puppyhood and socializie, socialize, socialize!!
Best of luck to you and your new guy!