I think the more important thing to note is his upper body.
You can see a slight counter rotation of his shoulders…he is preparing to rotate that upper body/shoulders in an explosive way. (That said, intentionally creating a big counter rotation usually creates other issues and may hamper velo, it has to occur as a part of throwing motion without being forced…if that makes any sense)
Still pictures are very difficult to use for mechanical cues. It would be easy to speculate that when his landing foot is on the ground his belt, his hips, will be rotated toward home with his shoulders lagging behind.
Mechanical cueing is tricky. The idea, as I am sure you have heard, is to create hip shoulder separation. The basic overview of this is the hips rotating then the shoulders…belt buckle toward home while the shoulders remain closed. In this picture his hips have begun rotating and his shoulders are closed. I would speculate a micro second later or two in his delivery and you would see front foot down, hips rotating quite a bit forward and shoulders just starting forward.
You may be opening your hips early, no one can say on here without video, slow motion preferred.
The creation of hip/shoulder separation is not an easy thing to teach however.
I am sure others will have more insightful comments.
By the way, if you are 15U and touching 84 you are well above average.