the toughest thing in teaching the curve ball is getting the 12 to 6 spin. the kid just sees what the ball does. When I coached i used to tape up the cans that tennis balls came in. The Circumference of the lid was about the same as a baseball. have the boy grip the can at the lid with the rest of the can as an extension of the ball pointing up. Have him throw the can with the spin of a proper curve 12-6. If he does it right he will see the spin of the can being 12 to 6. If not he will also see it.