Stop someone Publishing a thesis without giving credits to original author
You can't reasonably stop someone else from trying to publish something. 

You could however file your thesis right now (at ArXiV or a similar venue) to establish a time stamp. If then later your thesis is published under another name, you could ask the journal for a retraction. Be prepared to prove that you are not the one trying to get credit for your manager's work, though. 

If you do not want to expose your current version of your thesis (and you do not want to demonstrate a lack of trust by publishing an unfinished version of your thesis), you could make a PDF that includes your name and publish its hash value in a way that it gets timestamped (there may be other services for this). This allows you to prove later that you had this PDF already at the said time of publishing the hash value.

Note that using your text verbatim without your permission is probably a violation of copyright. Again, you need to make sure that you can later prove that you are the author and you have the text already today to make legal actions possible.

I'm not touching on the topic of whether your suspicion is reasonable in this answer.