OCD in children

Currently, I am working with a 6 year old student who has been diagnosed as having OCD.  We are working with the family and the child.  Frequently, he has temper tantrums.  We are still learning how to handle him in upsetting situations, such as, we have learned make a copy of his work and grade the copy (he can't even stand a sticker being put on his paper).  Twice, he has hit a student because he didn't like the way the legos or blocks were put away.  The other day, he punched the teacher in the chest because she wouldn't let him move his mat during nap time.  We are really struggling about where to draw the line and the parents are currently threatening to sue us because of the recent suspension for hitting the teacher (he was suspended for 3 days).  We want to help this child, but having adversarial parents who are using the OCD label as an excuse is confounding.  Any suggestions on how to help this child?