Research partner switching authorship before publishing to Arxiv?
Make sure this is actually malicious.  If so Contact arXiv.

For co-authored papers every author should be an owner of the paper. Usually, the person who submitted the paper gives the other authors the password. It sounds like that will be problematic in this case, but you can claim ownership of your paper without the password through the arXiv system.  (This will require manual intervention from someone at arXiv.)

On the contact form there you should probably explain exactly the situation.  They are going to be the only ones who can properly address and sort through what is going on.

Needless to say, you should make absolutely sure that this rogue coauthor is intending to do this and it is not some sort of mistake.  It seems like extremely odd behavior since the paper is already submitted with the correct authorship, so it's quite easy to prove he has changed the order.  Also, once you bring this to the arXiv's attention it could have consequences for this coauthor, such as getting banned from submitting papers, etc, so make sure the act really is malicious before doing this.