Jason,
I understand what you are saying. My first thought is that your son is rather young so you shouldn’t go overboard with this stuff. It’s gotta’ stay fun for him or it’s all for naught.
Regarding getting your son’s stride length to be more than his height, long strides are recommended by many these days. But Mills’ recommendation to reach a specific length (even if it is specified to be equal to your height or a certain percentage thereof) is, to me, a bit absolute-ish and we all know there are no absolutes. Tom House also recommends a long stride but he specifies that the stride should be as long as possible while still maintaining good posture and balance (which, in turn, implies you’re positioned to do other things well). I think that is a much more appropriate goal because it says that you still need to do other things properly.
Understand that it takes certain functional strength and flexibility to do things like maintain proper posture and balance, stay closed, get good hip rotation, etc. while moving faster into foot plant and, subsequently, release. Young kids often lack these attributes. Now, I’m not saying you shouldn’t work towards this goal. I’m only saying that you should have proper expectations in case your son can’t achieve it and that you might want to shoot for that goal in incremental steps. Also, it is good to put the focus on increasing momentum and letting the stride length happen instead of focusing on increasing stride length as focusing on increasing stride length can result in things like lunging and reaching with the front foot.
For increasing momentum, the NPA uses a technique that involves looking at the back foot drag line. The recommend focusing on getting the drag line to end two of the pitcher’s shoe lengths in front of the rubber. The only way to do this is to increase total body momentum which will pull the back foot away from the rubber.
Oh yeah, DM is a great guy!