Losing my mind

I am to be married to the man of my dreams in less than 6 weeks and I am worried that if things do not change drastically that our marriage will not make it. My fiance has a 17 almost 18 year old son who has been given everything his heart desires since he was a very small child. He has not had to work, pay for gas, pay for his vehicle/insurance/gas, clean up after himself or even take responsibility for his actions. He has stolen money, been arrested and banned from Wal-mart for theft, taken gas from our vehicles and is a proficient liar. Not to mention the things that have come up missing around the house that he knows nothing about. My fiance and I have been together for 3 years now and he has assured me for the last year and a half that once he graduated high school that he would go to college and live in the dorms. Well that was a sigh of relief until he botched up his senior year and we will be thankful if he even graduates in 3 weeks. I stressed a little over the fact that he probably would not be going to college and living in the dorm but was once again assured that a plan was put into place. It was even put into writing. He was to get a job, save money and move out with said date giving him time to save for all of this. Two months later, still no job and nothing saved. I was informed today that my fiance spoke with his mother and she stressed that his child couldn't move out that she would speak with him encourage college and she would put a plan into place. REALLY!!! She thinks she can magically have one conversation with this child and everything will be ok. Any thoughts on this matter as I am loosing my mind?

JJ, I should probably start by clarifying that the mother I spoke is my fiance's mother and is the teen's grandmother. His mother is no longer in the picture and has not been since he was a very small child. I am not living in a so called fantasy world in fact I am living this in real time. This is my reality. We have had to put gas cap locks on our vehicles including the lawn mower. We have to hide our keys at night so our vehicles do not go for a joy ride and put any cash on hand into the safe. We have had to put a pad lock on our bedroom door because he can't stay out of our bedroom. Often we home at night to find new holes punched in doors or walls because he can not control his temper. Trust me I understand that this is a crucial time in his life and that he definitely needs guidance from his family, me included. But when is enough enough when he does not care? The reality of the situation is he can either A. Get a job or B. Go to school. In addition to those things he needs to learn to act right respecting his family. Lying and stealing are not an option and will not be tolerated period. I am not looking for an instant fix. I am looking for some peace in our home and if that isn't an option for him then I say yes he needs to find another place to live.