Man you need to install some security pins (like the ones in the bottom of this pic). Basically you drill a hole thru the window frame from the inside (but not all the way thru. Even if he breaks the glass he will not be able to get these pins out from the outside. He would have to break the whole window frame to get in.

They aren't even noticeable from the inside, and you can pull them out with the key provided when you want to open the window.