Does my ex legally have a right to leave our son, with his grandparents because he’s working when I am available?

My ex husband and I share custody of our five year old son. New Years Eve is coming up, and even though my ex is working that night, he’s told our child that will staywith his grandparents instead of me. I am off work 1/31-1/2/19 and am readily available to keep him if his dad is working. My ex didn’t even call to discuss his plans with me or ask if that was ok. Our divorce decree states it is shared custody between my ex and and I, not his parents. Is he allowed to take my son to his parents for a holiday because he says, “It’s his day with Milo.” How can it be “his” day if he’s working? As Milo’s mother, should I at least have a say in this? This is not the first time something like this has happened.