Question about Growth --Giant Breed Puppies?

I have a male, 8 month old, Mastiff pup. He is 115 pounds, and very tall. I am concerned because he is taller in his back legs than he is in the front, meaning his back legs are about two inches longer than the front, so when he stands you can see that his rear is up higher than his shoulders. I have heard that it's typical for the Giant Breed dogs to grow unevenly when they're pups, but I'm wondering if anyone here has experience with this? I worry about my little big guy. Thanks in advance for your help.