Your manager is out of order.
If I was in your shoes I would go over his head and raise the matter as an informal grievance. If the matter is not resolved to your satisfaction I would then go for the formal grievance procedure. Whatever you decide to do, if you are in a union you MUST inform your union rep of what has happened.
Before you do any of this, obtain copies of everything and keep a diary. If you decide to speak to your manager about it before you take the matter further, make sure you have a friend with you when you speak to him or tape the conversation.
Good luck.