What makes you think he is a mix?
What you have to realize is that the GSD breed is very divers with specific lines that have different traits. And people mix those lines (which is still breeding a purebred GSD to a purebred GSD, resulting in purebred GSDs), and it is next to impossible to be certain that a dog that looks like yours is not purebred.
Just enjoy your dog. And learn everything you can about the breed and the different lines, and maybe at some point you might want to consider what different lines he possesses by his temperament, size, structure, etc. Still probably impossible though.
Jenna, RN CGC & Babs, CD RA CGC HIC
Heidi, RA CGC
SG3 Odessa, SchH1, Kkl1, AD
Ninja, RN CGC & Milla, RN CGC
Joy, Star Puppy, RN CGC
Dolly CGC & Bear CGC