It obviously won't matter either way if he is mixed or not! But remember, more than one male can father a litter, so it might be that that could have happened. Who knows really. The only thing that makes me suspicious is his ear shape, and just the way his face looks. And you are absolutely right, you can register dogs incorrectly with AKC as well, but atleast they require DNA testing for frequenly used sires, and they will DNA test puppies if it is in question. But I have gone to look at litters that are AKC and seen puppies that are not even close to being purebred, lol! He will give you all the love in the world, purebred or not!