Open his mouth and take a pic.
Also, he will keep putting on pounds and muscle for another two years. He will get more stout, but not much taller, in the same way we keep building up from the time we're 18 until we hit our mid twenties or so.
I'm trying to find a pic of a dog I had that was 100% certainly a golden / chow mix, because the owners owned both full blooded animals, and they got to each other and were caught together. Some of the pups look a lot like chows, some, like mine , looked like a golden retriever but with a black tongue.
OK, this is Stucky. He is the golden / chow mix. His mouth, gums, tongue everything, blue as could be. This chain btw, is 25ft, and on a 25ft runner. He didn't stay on all the time, just when we were gone.
I had one that was full blooded black show mother with an Irish Setter sire, both dogs with papers.
This is Lucky, she's the black chow / Irish Setter mix. She was in heat, so we had her in the pen, 40x40.