Basically means he needs to work on sliding his feet laterally quicker.

Too many players try and play defence with their hands and/or bodying them up.

Beat a player to point X by getting there with your feet..once your feet are there you've bet him, your chest is square and he cant go around you.

If you don't want to butt in, have your son ask the coach for a few YouTube videos of drills he could do in his own time.

Alternatively, if you frame it well, only an absolute monster wouldn't respond well to:

"Hey, thanks for the post-game suggestions for my son. He's really keen to work on it. Do you know of any good drills I can look up on YouTube that help with that sort of thing?"

'Defensive sliding' is only part of the story, depending on what the actual issue is.

Might be useful [to you] to find a book with diagrams of some specific footwork for different defensive situations [think of those old-fashioned dance charts].

Gets into the area of drop steps, pivoting into position under the basket, etc.

No point sliding [and holding with the knee] if the player has beaten him on the first step.

How old is the boy? Coaches are often guilty of assuming players know what they're telling them to do when quite frequently they have no idea.....and that's right up to pro level

Saying 'move your feet more' could mean a myriad of things. One thing I always find important, and Dante Exum displayed it perfectly when defending James Harden in the playoffs, is to 'tap' your feet when in front of the offensive player. When Harden stood around dribbling the ball 500 times Exum was always tapping his feet up and down to remain active. Once your feet are planted it is much easier for the player with the ball to blow by. I agree with the previous posters of gaining clarification from the coach as to exactly what he means

AKA +1

I always knew them as 'coojies' when I had to do them at training. One coach jokingly said that if any foot stayed in contact with the court for more than 5 seconds it would start sticking to the ground, so that I’d never be able to make a quick take-off.

The other thing to consider is whether your son’s body position and the foot his weight is on allows attackers to blow past him quickly. A body position which ‘encourages’ the attacker to go in a particular direction, together with the weight being over the leg furthest from that direction, should make it easier to take that quick first step on defence.

Go and buy an agility ladder and check out this link https://www.youtube.com/watch?v=67XP-AekUoA for some basic drills to help the kid with his overall footwork, balance and speed.

Other advice given, above, is all worth considering...

Agility ladders are good for eye/feet coordination, they are also good for improving a kid's endurance/fitness, they can definitely improve footwork, agility and balance, among other things that you can get from them the 'speed' term I used isn't straight line speed.

The speed, or technically speaking, the quickness of movement in your feet and speed with which you can adjust and coordinate your hands, eyes and feet in sort sharp movements can be enhanced using agility ladders.

The end result for a player is highly likely to be an improved defensive capability.

That's the 'speed' I was referring to, not how fast someone can run up the court, which of course is their linear speed, subject to their stride frequency, strength and power.

Replace 'speed' with 'quickness or agility' in my original post and it may make more sense...

Just to be clear, the OP is referring to a young kid whose body control and coordination is probably still in early development, the agility ladder can certainly improve these elements and result in better movement of the feet, which I believe was the major topic.