I have coached a variety of junior sports, over many years. In my experience one thing youngsters tend to do is (badly) imitate what they think they see the pro's doing.
For example, watch the world cup of Soccer, and you'll see the top players passing the ball with barely a flick of their toes. Kids try to do that, and the ball travels about 2ft.
The other issues is that the rules have changed. I am no expert on the rules, so this is subject to correction, but back in the day defenders were taught that they needed to set their feet to get a charge called. And in the same vein, a defender with his feet set who was bumped and fell backwards was likely to get the call.
So you would see a lot of guys with their feet set, and only moving their bodies and arms.
Now days, (and again consult an expert on the current rules and interpretations) defenders need only obtain position, and can maintain position by correct footwork.
A defender who uses his feet correctly can (re)establish position in an instant and receive a charge call. A defender who loses position and tries to defend by extending his knee or pelvis, is more likely to give away a blocking foul.
Furthermore, a defender who doesn't move his feet and too easily falls over, risks a flopping warning.
Best first step is to watch your kid play and try to spot what you think he's doing that is concerning his coach. Then ask for the Coach's help.
Even if you still don't have a clue, if you phrase it as "could you help my son with his footwork, let me know what we can work on" you'll get a positive response unless your coach is an a-hole. But try to avoid leading with "tell me what he's doing wrong" as that can be perceived as challenging.