My youngest was 5 1/2 pounds at just shy of eight weeks. She could sit in the palm of my hand. Today, at two years old she is still quite skinny, besides the fact that she eats 4-5 cups of high engergy food a day, and only weighs 53 pounds. She is however taller than her chunky sister who weighs 73 pounds. Her parents and her siblings today all appear to be of average size.
One piece of advice that my vet gave me that was fun to do was to take his picture every week by the same item. It is fun to go back through and see how much they really do grow. This will even be more helpful with your next pup as you will have a "measuring" stick. I did it by her raised food bowls. She could sit under them at one time, and now she eats out of them!
Enjoy your new puppy! I hope you get him soon!