Supervisor not approving submission because of coauthor he didn't approve of Okay I'm pretty desperate here.

I'm a PhD student. I had a cosupervisor (Prof Y) that my supervisor (and head of center, Prof X) removed (without consulting me) from my project and assigned (again without consulting me) another cosupervisor (Prof Z). The assigning of cosupervisors to a PhD project at my university happens with a form that both the new cosupervisor and the student need to sign, which I did not, and thus my supervisor (Prof X) thinks he's (Prof Z) my cosupervisor, but formally he isn't.

I have a publication ready that he (Prof X) needs to approve before I submit. His major issue with the paper is the fact that as a coauthor I have added my ex-cosupervisor (Prof Y), who indeed has scientific contribution. I received an email telling me that he has not been informed that I was working with my ex-cosupervisor (Prof Y), that this is not normal practice and that he (Prof X) won't approve the submission because of this.

Does he have the right to do this? Where is the academic freedom in policing who I collaborate with to such an extreme extent?

Please let me know your thoughts. I feel like I'm losing it.

UPDATE: I've contacted the vice-dean. He's going to look into the situation as I forwarded everything to him and tell me his opinions. He was shocked that my supervisor/HoC needs to also approve submissions.

UPDATE 2: Vice-dean has encouraged change of supervisors. He will support me throughout the process and will handle all economic issues himself. I'm arming myself with all the legal matters now. I'm making a meeting with prof X to politely explain that this is happening.

UPDATE 3: I changed supervisors. I lived. I'm so much happier. Thanks for the encouragement.
To learn at submission stage who else is coauthor on a paper is unusual, indeed. So to some extent, your supervisor being annoyed is understandable. To block the submission of the paper is a rather extreme reaction though. Could there be some history between supervisor and ex-cosupervisor that you are unaware of? (e.g. your supervisor suspecting the ex-cosupervisor of data manipulation, etc.)

My suggestion would be to apologize to your supervisor for the lapse in communication, and to ask about his reason for blocking the publication. Provided that this is indeed the case, it could help pointing out that the contribution of your ex-cosupervisor happened prior to him being removed as your cosupervisor.

If this approach fails, then the right moment to speak to the vice-dean has arrived.