Winter work for spring.
For twisting the wrist you want to fix that immediately or he will develop elbow problems. If he does it right the stripe will stay running 12-6 o'clockas it spins to the target. Just let him play catch with this ball and have him cocentrate on keeping the rotation correct as he throws. For the hitting use a double T and place one T in front of the other about 6in. Have your son drive the back ball through the front ball trying to keep arc of the swing level thru the contact zone.