I'd say he could still be pure, but there's always a huge difference between purebred dogs from BYBs and reputable breeders. Although some breeds have a wider size scale that is acceptable, for example, Papillons can be from five to ten pounds and I've met dogs from both ends of the size range. My Pap, Cali, is exactly five pounds and the majority have been seven or eight and there's a huge difference.
So, I'd just say he could be a purebred, just that his parents likely didn't fit the breed standard but were bred anyways simply because the breeder valued money over health and the breed standard. Although that definitely doesn't change how great of a dog he is. :)
Forever loved, never forgotten; my beautiful Indie. <3 Hoofprints on my heart.