Hi Dchan--great seeing you at Brighton! Sorry you couldn't join us for the Academy, but I hope you had a great time with your family and friends.
Your skier is having a little trouble in these uneven conditions. Here's why: he appears to start his turns with his head--tipping and turning it into the turn, followed by the rest of his body and finally his skis. This results in a sequential (first the uphill/outside ski, then the downhill/inside ski) and defensive "tails out" skidded initiation, also known as a sequential rotary pushoff. (It's especially visible as he initiates turns to his left.)
Sequential movements are usually a poor option in uneven conditions, especially when the snow gets deeper, because that one-footed "platform" is unreliable. And if it breaks away, it is impossible to push off from it.
More importantly, the "tails out" initiation causes the skis to skid sideways, which is especially problematic in these inconsistent conditions. It's a lot of work to push skis sideways in soft snow, and virtually impossible to balance smoothly on them as they grab and break away.
The solution is first to adopt an OFFENSIVE tactic for speed control--by completing glided turns, rather than using the skis as brakes. This will allow the skier to use the edges to hold the line, rather than as scrapers to scrub off speed. As a result, the skis will go the direction they're pointed, rather than skidding sideways.
To do this, he'll have to initiate the turn and steer with his feet and legs, rather than with his head and upper body. He should steer his tips gently and smoothly into and through the turn, instead trying to force the tails out abruptly. This movement requires RELEASING that platform that he currently pushes off from, and focusing on guiding the inside (downhill) ski down the hill to start the turn.
This movement is the OPPOSITE of what he's doing in the clip! It starts with the feet rather than the head. It turns the ski tips IN to the turn, rather than the ski tails OUT. And--here's the catch--it will result in an INCREASE in speed as he glides down the hill, rather than the decrease he gets by braking now. But it will give him more control of his line, and more ability to complete his turns, using gravity itself to slow him down at the end of the turn.
This is a big change! And it probably won't arise just by reading and understanding these words. A good instructor can help him find the key.