You can have his DNA tested now (something new!), which will tell you what breeds he has in his bloodline. It's around $150 or so I think. Check with your vet about it.
Wow.That looks so much like Mosin(my puppy from striaghttactical).Congrats on the adoption.Mosin is getting darker now and looks more like a Shepard every day
I honestly don't see a WHOLE lot of mix in Tanner; but if they are certain he's mix, I'm definitely going with Chow.
He's all Shepherd in the face, tail and feet. I only think maybe a little Chow because of his roly poly body as a pup in the first pictures you posted, as well as his coat.
Repped for saving a life from adopting at the shelter!