He looks thin to athletic. Probably if he was mixed with lab, he wouldn't be thin. They tend to carry weight with little trouble. But I couldn't say for certain that he is mixed at all.
As for some yellow of the hair, lots of whites have some shades in their coats.
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