Welcome to the board, Sherlock Golden. Actually, that is
something I've seen. All of our pups' waists did the same thing when they were small. I think it has something to do with muscle mass and lack of body fat--when they lean forward everything shifts forward and you get that skinny-as-a-rail look around the middle.
I think if he's eating and drinking well, pooping and peeing normally and is gaining an appropriate amount of weight for his stage of growth, there's not much to worry about. I'd just wait till his next vet visit and describe what you're seeing.
Welcome to the board, Lori. So when do we get pics of your new pup?