I think it's great that you're questioning how you and your child want and need to live! Both of you are worth taking however much time you need to make a decision about your future.
How can I know this is really the truth?
You can't. Recovering addicts are one day at a time. So are we on this side of the fence. We all have to learn to take each day as it comes, because the future is unwritten.
You could have already been married to him when this happened. You'd have to find a way to live with it or remove yourself from it. You get to do that now, too.
Your fiance' is no longer the same person you fell in love with. Addiction changes everything and that includes him. It's going to take time for him to learn how to deal with all the changes. He has new habits to learn, new ways of thinking.
Please take your time making this decision and please learn everything you can about addiction, check out a few Alanon/Naranon meetings.