Keep in mind you are only looking for 10-15" in stride extension. Thats a guess based on the angle. Thats the good news… I dont think you are looking at major changes.
A long stride is the result of previous “correct” movements. A lot of people are obsessed with long strides largely in part because of Lincecum & its the “hot topic” it seems.
The mistake is that they measure off 100-120% of heigth & put down a spot & say “get to it”. Next thing you know is that you have a “rushing” problem (too quick lower body) & the problem is even worse than the short stride.
ZC has the right advice on the drill… looks like you have used it. Revisit the drill & compare a dead on side video of your son & where Roy Oswalt or Randy Johnson are @ hand break. Youtube has great slo mo clips that will showcase this.
I think you’ll see Oswalt & Johnson have a good 8+ inches of hip lead down the hill & off the rubber before they break their hands. Your son, again tuff angle to see, appears to hover over the rubber & then lead toward the plate. Very common for youngsters.
That move right there gets you 75% “home” in my opinion. Add in the he is flat ground vs mound… the angle of the mound makes it easier to get early & longer stride momentum.
Last thought is to get him to think & feel that he is riding the inside of his right leg & foot for just a bit longer after hand break. Just a split second of more linear drive to the plate equals several inches on the stride. Im not saying push off… but definitely feel the ground force against the inside of the right foot & leverage gravity & momentum towards home plate.
Go one at a time on the adjustments starting with the Herschiser drill. Once you see results & “he gets it”, then advance to the next adjustment. Let him see the drill, comparisons & results on video.
I believe 86% stride length to heighth is the avg. without letting him know what ur up to measure of the distance of his stride before you tweak anything & then set up realistic goals.
I believe I’m quoting Roger but one of the best things I’ve ever heard re: stride length is that its a RESULT of things being done properly in the previous points to the kinetic chain vs the act of just getting out to 86, 95 or 105% of heigth.
Good way to measure the “effective results” is to look at the point where he initiates foot contact with his left plant foot. If he is still closed to the target with his shoulders & his weight is still back (draw a line from belly button to head… that line should be vertical or slightly towards the rubber vs leaning towards the plate) then he’s in good shape.