Had to add, I'd say on first glance a Chinook mix, but there aren't exactly many chinooks running around willy-nilly to breed plentiful mixes, lol!!
Also to clarify, the down ear has nothing to do with my guess. It has to do with the head shape, muzzle, the way the colors came out, coat type and texture, and structure of the shoulders in front.
Since you have his papers you should know exactly what he is, though. AKC or UKC papers are a pretty good bet at 'proving' he's pure. CKC papers are equivalent to a signed "I swear he is" note from the breeder. Just dig through your stuff and look! Let us know when you find the papers.