A pitcher’s timing with runners on is an issue for all pitcher not only 12 year olds.
The part of holding runners on is really a shared responsibility with the catcher, first baseman and the pitcher combined.
So here’s a suggestion that might help your son.
In the picture below I’ve made some assumptions based on competitive baserunners at the varsity level and beyond. And although this is a bit advanced for your son’s age, it’s worth trying with his club.
Assume that a batter can make it to first after making contact with the ball in about 4.5 seconds. This time by the way is a pretty good constant for varsity player with hustle. So, it goes to figure that if a batter runner can make it over 90 ft in 4.5 seconds, any lead off in feet from first to second will be less than 4.5 seconds.
In the picture below, I’ve made some assumptions for lead offs of 5 feet, 10 feet and 15 feet, with corresponding times to second base.
Now here’s where the catcher and the first baseman get into the act. That carved half circle in the infield is measured to be 13 feet from each base. So, using that as a benchmark of sorts, have the first baseman mark an X in the infield dirt at the 13 foot mark. Now, if the baserunner takes a 5 foot lead, there’s really no threat of stealing first. However, as the baserunner becomes more aggressive - with say a 10 foot and a 15 foot lead, now the pitcher has a problem. With those kinds of leads, the deliver to home plate and the catcher’s “pop” time, would be hard pressed to catch the runner at second.
When the first baseman sees the runner at that X mark in the infield dirt, a signal goes to the catcher which in turn alerts the pitcher to make his “pick off move.”
To see just how effective this strategy can be, setup a practice session with the fastest baserunners on the club, have only the pitcher, catcher and first baseman “in” on the move. Then tell the teammate to try and steal second. You’d be surprised at just how well this simple strategy works… .