I wish the dirt bike community had the same enthusiasm as I see at track days that provide free "coaching" for participants. One thing you may think about is finding someone at the track who looks like they are getting around at a pretty good clip and ask if they would give your little guy a bit of instruction.
I certainly would help a parent/rider out if they asked. Looking at your video getting your little guy to stand up is basic- as is when to transition from sitting to standing and back again. The position of the upper body, legs, and elbow position is hard to describe (You'll hear things like "attack" position etc, You really need someone to have him placed in the position you're looking for when he's on the bike up on a stand). As is positioning when entering a corner- from standing to transitioning to sitting (you just don't plop yourself down). Positioning and throttle control while going up the jump face and position in the air and landing. Corning position of the body and the lean of the motorcylce and throttle control exiting the corner. Of course braking and body position entering the corner- butt is back-slightly off the seat (sometimes just brushing it, other times a bit higher depending on the entry), but the upper body is kind of compressed leaning forward (like I said hard to describe) and using the front brake pretty hard at times. Along with standing up using the front brake is paramount.
It's all practice, practice, practice. I use to go around and around the same berm over and over again, or across a set of whoops. If there is no one around it makes sense to do one section over a number of times and then another section, and then put it all together. He'll learn much faster that way. I was helping a friend's kids out at a local practice area and we spent a lot of time (like 30 minutes) just accelerating out of one turn, standing, braking, sitting, and entering the next turn. And then moving to another section.
