Looks pure to me. Of course, mine also looked pure to me, as a pup. The consensus here was also PB. Alas, she grew up and is not. lol!
Without official paperwork, the only way to be sure is DNA testing. I used Embark for my shepherd and was very happy with it. That was last spring. They already have an even more updated test. I just ordered it for my other dog.
My answer would be "I don't know but it doesn't make any difference because he is an incredible pup. "Purebred" is almost a slang used by people who don't have a clue anyway. Why does it make a difference to them? I have had some beautiful GSDs, both imports and pups from imports (all SV titled). People would ask if they were a "purebred police dog". I would confuse them by saying "NO. He is a German Shepherd Dog with a Schutzhund III title and advanced tracking". All fluff but it annoyed me to question his/their genetics.