what a lovely forward going horse your on (sorry forgot if you said whether or not it was a mare or gelding)
It was quite a short vid, of only one rein so quite tricky really to add anything of any substance!
Anyway, position wise; An instuctor that comes to our yard is MASSIVE on postition and insists that if it not correct the horse cannot perform correctly, which is fair. So what I would say is, reins a little longer, so that would make you sit up a little taller (I think you tend to balance onthem a tad) hence they move quite a bit. When you rise out of the saddle think more of tucking bottom in, rather than swinging forward - keep hands still (as previously said with balancing) hands must be a little wider apart and MAKE sure you have your thumbs on top. Literally. Elbows must be at your sides at all times (not fixed), so think of the reins starting at the bit then going straight to your elbows, and your hands happen to somewhere inbetween.
So straight line from the bit to your hands - As with the lower leg, again as I said you seem to swing a tad your leg is a tad too far forward, as if you are pushing them forward to stand up. Think more about keeping the weight back through your knee (which will automatically push the weight in the lower leg. So don't think heels down (this makes you want to push the weight forward) think weight down in the knee, which will push the rest through the back of the lower leg.
The horse looked lovely and as he is young the things you really want to make sure is being is forward and responsive and indepant which he seems. I would consider some half halts though now, to gather up the implusion you have and to start to get him in a more relaxed and controlled temp/rythm as whilst he looked great, he seemed to me just moving forward without any collection or shape.
These are just my ideas and thoughts and I am not an ''expert'' so please forgive me if my ideas are of no use.
Goodluck though - x