Focus on your child...your fiance is an adult and as such is responsible to make either good or bad decisions.
You are putting yourself through alot of mental girations for no reason, it accomplishes nothing, it will not change the outcome..he will either use or not...that's the bottom line...it is ALL up to him.
Go about your life, get to Naranon or Alanon meetings, learn...knowledge is power...the true answer to recovery.