Let me start by saying that this swing is better than most in the age group. However, I believe he has an issue that needs to be addressed. The hesitation needs to be dealt with. He starts, pauses, and then goes forward. He needs to start and go forward. There shouldn't be a pause.
The lead leg is blocking his pelvis off. This is part of what's causing the hesitation, IMO.
Please take a look at this video of Teixeira. Notice how he does hesitate or pause. Notice how the leg begins to turn inward as soon as he starts moving forward. However, this isn't an active function of the rear leg. The rear leg is getting turned inward as the result of the rear scapula's retraction and the pelvis resisting.
If he has good discipline and knows how to foul the ball off, his mental mechanics are very good for his age.
Just find a way to eliminate the hesitation. That may entail eliminating the leg kick-for now at least. There are a few upper body issues as well, but addressing the lower body will make it possible for him to develop a more efficient upper body.