As much as it sucks, I think that you need to talk to your fiancee and tell him that at this time you do not feel that the two of you should get married. Explain to him your frustrations and explain that while you do want to get married to him, you will not be the pawn of his mother, and that if she does not get under control it may not ever happen. I would also explain how you were suprised/upset when you found out that he is still talking to his ex (that his mother wants him to marry), and that as much influence as she has in his life, you are concerned that he might go with her on that idea. It is better to call off the wedding now, then to realize a couple months after the wedding that it was a bad idea.

As for your fiancee being concerned that his mother will be ashamed if the wedding is called off after she sent invitations, tell him that she should have thought about what her interfering would do before she turned into FMIL-zilla