Such a good thread you've started, it highlights so many important things we should all look out for.
First, those DNA profiles are fun but should not be relied on for determination of breed. Many a known purebred has come back with mixes in the profile. Most pure breeds did have some other breed mixed or introduced way back when and those other breeds can show up in a profile. Some DNA profiles are an outright scam. some require a photo of the dog and you have to wonder why. You CAN rely on DNA to determine if the said parents of your dog really are it's parents.
Second, the breeder investigation is the most important thing you need to do when searching out a purebred dog and you must, you must, you must do it BEFORE you get the dog. Each breed will have a breed club that sets the standard, explains health concerns in that breed, clearance tests that should be done before a dog is bred, life span, often tracks competitive awards. some tests we have gene tests for, others, like good hips and elbows, all we can rely in is that the parents had good ones as per Pennhip or OFA. Most breeds require eye and heart clearances. Some breeds have peculiar ailments common to only them.
Conformation competition will compare the dog's structure to others of it's breed.
Structure is important for a dog to do activities it may have been bred to do without undo risk of injury. GSD and Malinois often compete in Schutzhund. There have been some of each breeds in Obedience, Rally and Agility trials I've been to.
Sadly, you need to check with the registering agency that the dog parents you are shown really are the dogs with the AKC or CKC registration or the hip or elbow clearances, etc.. And you have to watch that someone claiming CKC registration means Canadian, not Continental. If you want to compete in activities where such titles are tracked by the registering agency then not only do the parents have to be registered but your dog does too.
It sounds like you have fallen prey to a Backyard Breeder. A BYB. Your dog looks lovely. Does his possibly being a mix (I'm not sure it is clear he is a mix, I would not rely on that DNA test myself) prevent you from doing the things with him you had planned? Which did you want, the GSD or the Mal?
Sadly people deliberately lie when they sell their car privately and say it was never in an accident. You can check such things these days. BYB can be hard to suss out because many are good hearted but naive dog lovers who don't know what they are doing harms the breed. Not all are deliberately scamming you. Some really think just because they love their dog she'll have great pups, not even knowing to look at the horrible elbows she is passing on to unfortunate pups.
No, we were fortunate to know several people involved in our breed and knew the checks to make before we settled on a breeder for our pups. sometimes you can approach a breed club for an evaluation of your dog and sometimes a kind of registration number that allows you to compete in performance venues can be issued. Good luck, I know you love him to bits regardless.
PS: There are a couple of BMD in my village and one is a champion show dog. Your dog does not have the coat of a BMD at all.
http://www.ckc.ca/en/Choosing-a-Dog/...e-Mountain-Dog