Movement in stretched position only

I may have something similar. It's close enough so that I want to "share". I'm T 9 incomplete, so that part would look, at first glance, like it wouldn't relate to his condition. But being 'incomplete' can make for some interesting combinations.

I have only one leg. I lost the other one at the moment that the car hit me - and broke my back, so when I mention my "leg", that's the reason I won't be saying "legs". Here goes,...if I'm sitting up and my leg is bent, but I want to straighten it, I can usually get it to straighten out by laying back in bed. Voila! The leg straightens. Not only that, but it 'stretches' out board straight. There's even a secondary reaction where my stump will immediately follow the full leg with a little mini-stretch of its own.

A physical therapist, the best one I had, felt of the calf of my leg once and mentioned that my leg isn't "flacid". But I assure you, I cannot control my leg. I can't bend it or straighten it. However, by giving it a little push with my hand and then manipulating my upper torso, the impression to a casual observer would be that I had "straightened my leg". One rude hospital aide even loudly and triumphantly announced to her co-workers, "She can move it! I saw her move it!"

I can't do anything else with it, not even after it is stretched out. So, at that point, my condition may be dissimilar to your son's.