sowilson,
Its understandable that your son does not feel comfortable throwing his curveball. His two seam and four seam are similar to the hand position he has when throwing across the diamond or whatever other positions he plays. So naturally those are his most comfortable and commanded pitches. The curveball requires a different hand position but body mechanics dont change. I'd imagine something looks entirely different when he throws his curveball, head tilt, upper body sinking at release, etc... Rather than go into a pitching lesson on curveballs, I think we can get him on track with a better mental game plan. Most kids have a general idea and game plan on their fastballs. They dont think about the shape of the pitch when they release. There is no thought on their fastball from fingertips to catchers glove. Its more focused on getting a good delivery, focusing on the target and letting that sucker fly. On a curveball, pitchers tend to think less about what they are doing in their delivery, less about a target and focus more on what the pitch is going to do from fingertips to catchers glove (shape). Tell your son to throw his curveball like he is throwing his fastball. A good fastball delivery and focus on a target - forget about the shape of the pitch. Remember its not the size of the break that makes a curveball effective, its how similar it looks to a fastball that makes it nasty. I have worked with several kids on trying to throw their curveballs as straight as possible. Sounds weird but it works. A good way to work on this is to have him picture a hula hoop about 5 feet in front of the catcher. Try to throw the curveball straight until it reaches the hoop then allow it to break once its goes through. This will add depth, late break, and deception as his delivery will look like it does on his fastball. Make sure he is staying tall at release - head up, ball down. Hope this helps!