The towel drill teaches you to extend out over your stride leg and release out front.
If you have a big stride it is harder to fall off until after ball release.
Usually short strides result in falling off and a higher release of the ball. This usually leaves the ball up and more hittable.
The T drill must be done properly so that you have to extend way out front.
I tell my son he would never reach the target doing the towel drill with the bad habits he has developed. The target can be the glove of another play set up at an appropriate distance on his knee or a chair.
You should read up on this drill and then get your son to repeat it with a ball inj a bull pen. I used to stand in front of him so that if he turned off he would run into me. It is a tough habit to break.
Another thing is I like a high post leg kick before it swings around landing squared up to the plate with head low and focused on the plate. I want his momentum to be towards the plate and fall towards the plate. Don't rise too quickly. When you fall off you are opening yourself up to a line drive that could take you out let alone not being the 5th infielder.
I agree with AL MA