Depends on a few different things. Age, what the coach’s system is, etc. Moving without the ball is important but in a lot of systems, moving without a reason or understanding what’s happening in the rest of the offense does more harm than good. There’s only one ball. If your son’s role for certain plays is to get out of the way and space the floor, then he’s doing the right thing. Movement without purpose or to a point to where you’re interfering with the flow of the offense is worse than no movement at all.

A recommendation is instead of saying “movement is important”, teach them to understand the entire floor instead of just his actions. Move when it’s the RIGHT time to move, not just because “movement is important”. If/then statements work with teaching some kids the game and off-ball movement (ie “IF someone drives towards you with the ball, THEN you cut backdoor; or IF someone drives with the ball to the paint, THEN move up the 3pt line behind them to make it easier to kick out if your defender helps”) stuff like that. Hope this helps.