Could his stifle/mincey gate in the rear be because he is behind the bit?
I don't believe so, there is a real stiffness to the movement that seems to originate from the stifle. Horses can be ridden behind the bit will look similar in that the stride will be short but this guy does not have the freedom of movement from the stifle area that I would like to see.
As for the free jumping, it seems to me like they have pushed him to jump obstacles before he has developed any kind of technique. This has not been helped by the way he is instantly checked/overbalanced upon landing.
Have a look at the way he jumps - there is no clear take off, airborne and landing phase, he jumps almost
simultaneously with his front and rear end. What you want to see, is a clear take off - thrust from the foreleg through the centre of the horse so that the front is higher than the rear with the hind legs flexing somewhat, supporting the weight of the horse for the departure.
In the airborne stage the head may drop some, there is some rounding of the topline (i.e. bascule) which allows the forelegs to be tucked neatly under the chin of the horse.
In the landing stage the front legs should land clearly first and the hind legs should still be well tucked. As the forelegs begin to support weight, the body begins to rotate back to its horizontal form and takes begins to stride as the hind legs land.
Here is a video of nice technique over free jumps, not perfect but you can clearly see each stage, compare the jumping stages of this horse with the one you posted:
Really he should have been free jumped in a chute type set up as in the horse above for such obstacles, doing it on the lunge line is very bad practice, not safe for the horse and not good for his long term health (i.e. in the upper neck/poll area) or his training.
Sorry to be so harsh, don't take it personally! He looks like he *could* be a nice prospect, but there are a few issues that I would want sorted out first.
ETA: I just re watched his video and it isn't as bad as I first thought, plus the jump is pretty small so take the above with a grain of salt