This is mostly for my son, although as with most rec teams I've got several kids who swing this way. My son has a short stride, good hip action and although he lunges on occasion he doesn't usually lunge. If you look at the clip of Gwynn posted by swingbuster, at the point where Gwynn has the knob to the ball and the bat parallel to the path of the ball my son would have his shoulders fully open and his hands out in front of his body, with the bat in the same orientation as Gwynn. From that point he has to leave the bat head back on low or outside pitches and just keeps the shoulders turning to get the bat around on inside pitches. I believe he got this habit back when he was 9 or 10 to make sure nothing ever got by him when I was throwing BP too hard. Whatever we do it will take quite a while to fix because he's had a lot of reps doing it wrong.
I did work with several of the kids on the one hand drill with the lead hand to show them how much they are opening up their shoulders.