While working with one of my students today I remembered another things. Have your son work on his belly. Put a clipboard with his paper on the floor and have him lay down on his belly propped up on his elbows. It forces a proper grip and increases core strength all at the same time.

Standing helps for directionality.....up is up.....down is down.....drawing large helps with crossing the midline.....writing requires part- whole learning of the basic lines....I would still use small pieces of chalk on the easel

I agree short pencils, about the size of a gold pencil helps also triangular pencils.

I bought a pencil gripper for my grandson from his OT. It forces him to hold the pencil the correct way.

Verbalized basic lines as he makes them......use in patterns... C o a d g q s..r n m h..etc