I worked with my son's change up all winter long. Finally he has confidence in it. We fooled around with many grips and tried other things.(dragging foot , dont do it) and finally he has a comfortable grip. ALL I can say is to throw the change up during warm ups, throw long toss with it, throw into a net, before games. You have to get comfortable with it to the point where you aren't thinking about it.
Last night, during a bullpen I told him to call his own pitches. Changeups and fastballs only. Needless to say he was smiling all night! :lol: The change-up looks like a fastball then it drops a bit across the plate. Before he had issues switching from pitch to pitch. If he threw a two seamer for the first pitch the change up would hit the dirt the next. Curveball one pitch and the change up would hit the dirt the next. All I can say throw the pitch until it is second nature.
Confidence is the key. IF your kid has confidence in it he will throw is effortlessly!