I would suggest that you not have the pitcher focus on keeping the toe on the ground until release. I'm also NOT suggesting that you tell him to get it off the ground either. I wouldn't focus on that component at all. It's barking up the wrong tree. The action of the back foot will happen as a RESULT of good weight transfer, rotation of the back leg and foot, hip rotation, shoulder rotation and forward trunk flexion. Keeping the toes down is a result not a cause of anything.
I've looked at many videos on this and the only one I've ever seen who lifts the toes early is Schilling. Anyone else have any video of another? All I've seen have the toes touching because they are being dragged a bit. There are those out there who recommend that you should keep the toe down for balance reasons. I'm not sure about this one because it seems that there isn't any weight actually being borne there.
What drives me up the wall is when I see books or other coaches getting kids to do a drill where they put an object in front of the rubber and get the kid to focus on getting the back foot over that object because they see dragging the foot as a bad thing. Where did this come from? Have they ever looked at major league pitchers? Apparently not or they wouldn't be doing this useless and potentially damaging drill. All this will serve to do is get the upper body moving forward much too soon.
The healthy way to look at this is to get him to fire the front hip at the target, sideways, rolling the back foot over onto it's instep and not lifting the heel early. Then, as the weight has been shifted nearly as far as it will go depending on the stride length, rotate that back foot and entire leg around it's longitudinal axis (don't let the knee fly outward), rotate the hips INTO LANDING (not after like you'll see in various placs). Keep the shoulders closed during this to set up the hip/shoulder separation required to stretch the muscles and connective tissues of the front of the torso. As the front foot lands, fire the pecs at the target while rotating the shoulders as violently as possible. The elbow comes at this point also, in conjunction with the shoulders. Continue the forward trunk flexion through release to a nice flat back finish. This will cause the back foot to come off the ground.
It's all of this stuff that's important not whether the toe is off the ground and instant before or after release. It's a result not a cause.
I hope this helps.