in the most recent video you can see his feet flatten and toes spread wide out , the side shot show his back shortening , tensing over his loin.-- see previous comments about movement affecting back muscles.
I pay a whole lot of attention to the front where it all happens (or not) .
Can you do more side views or a casual side stack to see what his front assembly is like to see if this is due to an imbalance of front to rear .
what can you do -- have the dog looked at by a canine massage/chiropracter/accupuncturist. If you were closer I could show you points just above the pelvis where you can touch with slight finger tip pressure and see muscles flick , just like the high school biology class where you stimulated a dead frogs muscles with mild electrical stimulation and got that flick.
muscle use has motor memory and he is young and if tension is released he can be retrained.