He hasn't 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.
Ahh I see I thought the last picture was a pic of the father. Thanks for clarifying that. If you like the pup and it is healthy. I say go for it. He may be one of the best dogs you ever had. The same thing happened to me with Taz. The guy that gave him to me 14 years ago. told me both parents were GSD. Through the years the vet told me there was Chow in the wood pile. Taz ended up living 12 years, pretty much problem free. If you are looking for a German Shepherd., seek a reputable breeder; then you will know for sure. If you do not want to do that, then you get what you get.
Big gamble for sure.
"A House Is Not A Home Without A German Shepherd"