It's impossible to diagnose over the Internet. From your description I would suspect a structural problem with his spine and also possibly HD. I would be very surprised to see DM in a puppy. My first gsd was diagnosed with severe bilateral HD as a 10 month old puppy. At that point it was very clear that her hip sockets were malformed. She slipped, had trouble on stairs, etc., got tired after a lot of running and heavy exercise but structurally she doesn't sound like your dog.
I would definitely want an x-ray from an orthopedic vet to see what's going on in his spine and hips. I would get that immediately because there is almost certainly rehab you can start and supplements that could help.
Do you leave near a teaching veterinary hospital?
Ruth & the 4 Legged Rescue Gang
Rafi the malaroo
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