Tips

Seat time, seat time, seat time....and get away from the track for a while.  Go practice figure 8 drills, and turn tracks if you can.  Working on fundamentals until they become second nature is the best way to improve.

There are no shortcuts. Bike set-up is the most important thing. Correct springs and sag are a must. Technique comes next, take a class that teaches proper technique or go online and find videos that teach it. After that, it's all seat time. The more you race, the quicker you will improve. Also, practice starts, it never hurts to start a race in the top 3.

And don't worry about a younger kid being faster, it's going to happen to you no matter how fast you get 😄

These are all my opinions, take them for what they're worth....

Let him have fun, if he wants it, it’ll grow from there.

Tips? Frosted.

Watched the vids.   Tip #1 - never ride with your helmet not buckled properly.

If he can have fun while working on improving technique the speed will improve naturally with seat time. Trying to go faster without sound fundamentals is a recipe for injury. Maybe a few riding lessons could be beneficial?

Sounds like an excellent base of knowledge to put into practice. No substitute for practice! Maybe take some videos of him riding so he can see for himself what he’s doing right and what he can improve. 

Lots of ways. Got to be comfy with controls. Starts, stops, corners. 

Lots of vid, then compare with faster riders, then he can see what he's doing.

I watched the videos. I'm far from a riding coach or mx analyst but since you asked here ya go. These are things I've read or had people tell me over the years.😃  (Disclaimer)  I'm a 66 year old novice so....... maybe this will help maybe not lol.  My biggest advice is make sure it's always fun, like others have said make sure to go out and just fart around and ride for fun too. Don't make every ride a lesson. Just my $.02     

Starts.  The start is 2 parts, getting the jump off the line and being first coming out of the first corner.  When you line up imagine the line you want to take through the first corner, know exactly where you want to be going into the corner, inside, outside, middle, whatever, just have a plan on how to be first out of the corner beforehand. Visualization is the technical term, think beyond just being first out of the gate, have a plan. On getting out of the gate first. Don't stare at the gate for 30 seconds waiting for it to drop. Wait until the last few seconds before the gate drops, wait for the card to go sideways, or if it's just one starter wait until he's almost back in the doghouse, then concentrate on the gate and explode the second it flinches. The thought behind this is that you can only "Super-concentrate" or hyper focus if you will for a few seconds, if you start staring at the gate too soon your mind will wander, you can't help it it's just how we are. Wait until the last second to look at the gate, hyper focus and explode the micro second it flinches.  

Corners. Get in and out of them as fast as you can. lol.  Don't coast before the corner, lots of guys start slowing down way before the corners by coasting. Brake as late as possible, don't go skidding into the corner like a wildman obviously but don't start gradually slowing down 30 feet before the corner either. Train yourself to make braking markers, "Ok this lap I started braking here and had plenty of time to slow down", next lap brake a little later.        

Straights. Treat every straight with same intensity as the start straight.

Jumps. I've got nothing besides try and get someone to tow him over. The only doubles I ever did were ones with cheese.  I do remember seeing an article by Rick Johnson that had a formula for jumps on a chalkboard, Speed + height = distance.  I guess the theory being it's easier to gage how high you're going to go off a jump than how far ?  So if the guys clearing a jump are going 10 feet higher than the top of the jump, go off the jump fast enough to get you 10 feet high, then, in theory you will go far enough to make the landing ?  I don't know can't jump for beans and never could, but maybe that will help him. Good luck have Fun !🙂

Motocross is such a tough sport with this, you don't just sign your kid up and have coaches, facilities, and equipment provided like with school sports so it is tricky. I'd say the biggest thing in addition to getting him to some riding schools is finding some good, fast older riders for him to ride with regularly in your area if you can. The biggest impact on my riding came from riding with my dad and his friends who were fast A riders when I was a kid. I wasn't worried about them blowing by me or anything because they were on big bikes while I was on a 65 or 85 but I watched them and rode with them and modeled by riding style after them, picked up on how they rode, took tips when they gave them, and just generally absorbed a lot. I think there is a sort of osmosis than can occur when you ride with other good riders especially when you're young so hopefully you can find a group to join.