Tingling is from some form of nerve compression. If it persists stop throwing and go see a Doc.
If the tingling goes away but the forearm tightness persists try this.
Before and after throwing (but before icing) take hand lotion and apply it liberally to your son's forearm, from the elbow to the wrist. Now you have him face you and, starting at the elbow and moving down to the wrist, you grab his forearm with both hands, one above the other and apply pressure like you were giving him an " indian rope burn". Move down the forearm to the wrist.
Repeat several times. Now ask him where on his forearm the muscle tightness eminates from and take one of your thumbs and apply pressure to that spot. See if you can feel a knot and slowly increse the pressure. Move you thumb back and forth in the same direction as the tight muscle . Get him to take deep breaths and try to totally relax those muscles. Repeat the 'Indian rope burn" motion several more times.
What you are trying to do here is take a muscle or group of muscles that are spasming and break the electrical impulse that is causing it.
He should also stretch his forearm muscles before and after throwing.
Let me know how this works for you.