It is always a shock when we find out these things about our loved ones. I have helped in a rehab program for men and taught family recovery. One good thing about this is he did come to you on his own to let you know how he needed help. It is also good that he put himself in treatment. I am not sure where you are spiritually but my experience with myself and others I talked to is take a day at a time. Don't try to figure things out and it will be hard to trust and it will take time but give it all to God and allow Him to provide and give you the strength to get through what is going on in your life with the new baby and all the sudden changes. I have seen many families that the wife made it through these difficult times. It is a painful time and he loves you and the baby. In with my experience with these issues he is sorry for what he's done but it is wise to be careful and concerned. Learn what you can about addiction and understand it is almost like a sickness. Take care of yourself and your baby and believe God will get you through this.
It is good that you have a place here to express your feeling and share.