Congrats on all of the work with your son! I've gone through a bunch of the old posts...it's been great watching him develop over the years.
Here's a video of the 11 year old version vs the now version:
To me, the biggest differences I see is that he seems to be reaching out with his front leg some now, when at 11 his drive was mostly back leg driven and more explosive burst toward home. At 11, his back leg was driving his front leg, and his front leg stayed closed a bit longer then, and seemed to have a more powerful unloading hip/glute usage...did a very good job "weighing more as he's moving forward." He also seems to have an earlier shift forward on his back leg/hips when he was 11. [Additionally, when he's bracing up now, his front leg is pretty deeply bent, and at 11 his front leg seemed to be in a stronger position.]
Baseballthinktank had a good article on this and has a bunch of good drills:
Obviously, either CoachXJ or BaseballThinkTank (or many others) can help much more than I can, but just thought I'd at least provide a comparison video.