Get the dogs DNA checked, we done this with ours, as we and the vet thought he was Corgi and Chiuaua, DNA states he is 28% Yorkie, 26% Chiuaua, and the rest all strains of terrier.That is correct and also where we have spent the most on our Yorkies no matter whether we brushed their teeth or not. Most small dogs do not seem to like having their teeth brushed.
If we do registered him as a mongrel OH will probably give me and earful plus of course any claim could be rejected due to fraudulently registering the dog as a mongrel. In the EU and US the Biewer is a recognised breed, but not in the UK as Kennel Club sitting on their hands! Both his parents have pedigree papers with Poland kennel club!