at first I thought your dog looked just like mine, until I read how old yours is. When my dog was a puppy, she looked a lot like your dog. Here she is, when she was about 4 or 5 months old:
Now, she's 10 months old and over 80 pounds, so when full-grown, she may be twice the size as yours! Her dad was a dane, and her mom was a black lab/doberman pinscher. So.. maybe yours is a dane/black lab/minature-pinscher! LOL! That would be interesting mating...
Beautiful dog, though!