I think his neck may look a little low in the picture due to my lack of knowledge on how to properly stack a dog
While this is part of it (and it takes time to learn how to #1 stack a dog and #2 learn how to stack a dog based on its individual virtues and deficiencies), when you start learning how to look at the skeletal structure *inside* the dog, and not just the outer shell, you can see the issue better.
Even if you brought his head up a bit, you would be able to see that where his neck ties into his shoulders, the set is a bit low.
Do you think his topline is off because his hind leg is a little too stretched out?
Again, yes and no. Regardless of how you stack him, this puppy does have a very steep croup. You can make it look better or worse, depending on how you stack the dog.
Will his croup change? Probably a little bit. Will it ever stop being steep? No.
His pasterns should firm up a bit as he ages. He is still just a baby after all
His topline will likely do all sorts of weird things as he grows, and his underline will follow suit
I actually took pictures of my little girl from week to week until she was about 6 months old, so I could see all the changes she was undergoing.