Dano,
Tbe fix depends on the cause and that's what you have to figure out. Unfortunately, distinguishing cause from effect can be difficult. However, there are things you can focus on and experiment with.
Some things that can cause a lean to the glove side include:
> Simply trying to use the head and shoulders to generate power - fairly common among young kids who don't understand power comes up through the body.
> Instruction such as "get on top of the ball" or "throw over the top" can cause kids to lean to the glove side.
> Starting position on the rubber and/or stride direction can affect posture later in the delivery by causing the body to lean just to get the shoulders squared up to home plate if the body has tracked offline from the target.
> Postural issues early in the delivery such as leaning back (towards 1B for a RHP) or leaning towards home plate can lead to falling off.
> Dropping the glove or flying open with the glove can affect posture or pull the shoulders open early resulting in leaning/falling off.
> A lack of tempo and momentum can affect balance and posture - like riding a bicycle too slow.
Try looking at these things and experimenting with adjusting them one by one until you find the one or two things that causes the others.
On final comment... Pay attention to when your son falls off to the side. If it's before ball release, then it deserves looking at. But it it's after ball release, it's likely just his residual momentum and follow-through pulling him around - probably not a concern unless it leaves him out of position to defend himself against a come-backer.
By the way, the towel drill is simply a drill that lets you practice putting it all together. To get out front far enough to hit a target positioned at stride length plus 5 heel-to-toe steps, you have to put it all together. Posture and balance have to be intact. Glove-side has to be managed well to avoid posture and timing issues. Tempo and momentum have to be good. Mess up any one of these things and you probably don't hit the target. Hitting the target isn't really the goal - it's feedback. The goal is to have optimal mechanics and timing.