While I understand what "DodgerFan21627" is getting at when he says, "He is lacking separation between upper and lower body which means he's losing out on some power", I have come to use the term "loading" in it's place, as "separation" IMO, is simply a "result/effect" of proper loading.
So, in your son's swing, he has no really loading of any type around the rear leg (lower half), that would, in itself bring the hands/torso/upper half (referenced in several different way) rearward.
Once he has loaded the lower half (while the upper half is still "loading") and begins to open the lower half to the incoming pitch, he will be creating a "stretch" along the rear torso ("axillary lines" for those so inclined)......so you now have the lower half turning forward with the upper half turning/remaining rearward......hence, "separation".
Separating the upper half from the lower half, or "the hands from the rear hip", and not "the hands from the lead leg/foot" as you'll often times hear.
Something like this.....
If you look quickly, I think you can see him "step-in" to close/coil the rear hip, and as he does, his upper half also turns rearward ("effect", not "cause"....ie. don't counter-rotate or turn the shoulders independently)......as the pitch approaches, his lower half begins to the ball as he keeps his upper half/shoulders from moving, until they are eventually pulled/turned into the swing by the now rapidly accelerating lower half.
Yes, I know.....that was a lot to take in. So to make it easier in the beginning, have him start by, "going backwards to go forwards" (using his legs and hips, and not his shoulders to turn rearward), then work from there.
Best of luck,
In memory of "Catchingcoach" - Dave Weaver: February 28, 1955 - June 17, 2011