IMO, there are no drills to teach this, it's a case of, hopefully learning by doing, playing games, remembering when you made the right decision and realising when you made the wrong ones. Learning by your mistakes, recognising the signs around you that led you to make that (right or wrong) decision...it helps if the players around you know what they're doing too and don't drag you out of position...but that's a big ask at 12yr old level.It came from my 12 Yr old being asked to play Defensive mid and being told to be disciplined during a game and to work together. It's confusing for a kid, and older, though, when to press, when to hold your position, when to go with the runner, So just trying to come up with some of the basics really
Players on £100,000 a week can struggle with this - or so it seems.
First off, Is your lad(s) 'a natural defender' or does he always want to bomb up front or up and down box to box?
If he's not mentally suited to defensive Mid F. well we all know Sq pegs in round holes are no good for team or player.
Is he a dribbler, a passer, a shooter or a runner? Because dribblers etc, will always mostly revert to dribbling, the passer will pass and the shooter, especially the head down type, their first Instinct is to shoot.
Not really what you asked for, but that's all I got