The OT at school might be able to give you some hints for how to help your son with his writing. If he responds to games (which he might not), you might try doing some sky writing where you swing your arm around in the shape of the letters. Talk your way through the shapes ("start with a circle, add a stick"). Some teachers use shaving cream on a table top. Children write with their finger in the shaving cream. If he has problems with finger dexterity, I suggest practicing with large muscle groups first (sky writing) and working down to the smaller muscle groups (table top writing).