How can I get my fiancé to realize that his mother is financially taking advantage of him, and that it is causing us to not be able to start our lives together?

How long have you been together? How is she taking advantage of him? More info please?

I would set him down and have him show you where all the money is going. Once he has done so, say something to the affect of I didn't realize we were giving her so much money. Perhaps we need to talk to her about figuring out how she can help herself because we really could use a for instance family weekend out or a mini vacation, etc.