If you haven't already, you should report him ASAP, so that if he succeeds your university will know he copied you and not the other way around. A record of you having lodged a complaint first would make your story more credible if it came to it.

Report him for plagiarism and do it immediately. Otherwise, you could be the one that comes off looking like a thief, and gets kicked out as a result. And getting booted out of a program due to plagiarism accusations makes it near impossible to get into another school.

You want a time out?
You better spit a rhyme out
Before the community of real mc's die out
College students say to me "you ain't smart"
Record label A&Rs say: "this ain't art"
These are the contents of the covenant of the art
Listen to my chest beat tell me this ain't heart
You gotta be as obsequious
As the Disciples of Jesus
This is my MASTER THESIS

Oh, I guess not many people like intellectual music.

Sounds like the kinda relationship you'd normally have with your parents

Report him to your professors for plagiarism. Then slam his fingers in the desk drawer.

To be fair, I read and used the theses of 4 former students from my lab to help me with layout and finding sources for my thesis. Given the fact that no master's thesis is on the exact same topic, I highly doubt your ex roommate would be able to copy it (I can't imagine your advisors approving the same topic, especially not in the same program at the same school). The FML here is the fact that someone was going through your shit, which is not acceptable at all.

There's a difference between "former" and "present". Former ones have been used already and are in the system. Present ones won't be until the time for them to be checked comes around. And unless its changed, some students do get the same or similar thesis. If they didn't, then using another's paper wouldn't be a problem but some students have been kicked from universities because someone else used their thesis.