Its hard to say at his young age. He very well could be purebred. Like I said he looks a lot like his mother. He looks shepherd to me. If you seen the parents and the breeder is telling you they are shepherd, and they look shepherd. What is to be questioned? Even if he is mix, it is not much. Hes is a handsome little fellow!
He hasnt seen the father? I think thats what making him question if the pup is purebred. The breeder just told him the father was white and probably provided a picture.
One way to know is if the dog is registered with the AKC and if the breeder gave you the paperwork. If not, your probably like most of us BYB dog owners. We can just guess and go off what we were told.