My female GSD mix exhibited signs of HD early, not nearly as young as your dog though. I think she was about ten months old before we realized what was going on.
Some of the clues we saw were lameness after intense exercise, limited range of motion and signs of pain when we moved her back legs, and struggling to get to her feet after she had been laying down (her front legs doing all the work of getting up). We could also so a marked difference in the way she moved when we compared it to our other dog, kind of hard to put into words, but Niko's movement was much more fluid and effortless.
I don't know a whole lot about puppies and the awkward stages, but I think I would just wait and see if I were you. The only way to diagnose HD is with an x-ray. If you wait to get him neutered until he is a year old, you can ask for x-rays of his hips (and elbows if you are curious) to be done at the same time while he is under anesthesia. That should give you peace of mind if they look good, or alert you to a potential problem.
The biggest red flags (to me) are pain and lameness, and so far you aren't seeing that, so I would just wait and try not to worry too much.
Leah: Newbie dog owner
Niko: American Showline GSD 4 years old
Rosa: American Muppet Dog (GSD/Border Collie mix) 4 years old