i have custody of my son and if his mother is a week late picking him up for summer vacation can i refuse her visitation

If the visitation is in a court order, you cannot voluntarily violate the court order.  Violating the court order, can lead to contempt and if found guilty, leads to mandatory jail time.  You cannot change the court order, but a judge can change it.  If you have the grounds for modification, you would need to go back to court to modify the order.  I would suggest contacting a local attorney to discuss if you have grounds for modification.