I think you are correct when you say your son needs the stability and each time he loses your fiance it creates fear and panic. But your fiance needs to make a choice...your son has some issues and they won't just disappear because he is there full time either. So he needs to decide whether he is in for all of it...the good and the less good. Boundary setting can work well if done with love and care and not harsh criticism or punishment. I also hope that your son is seeing someone regularly so that he has a place to process all of his feelings...not just about your fiance but all else that goes on for a 13 yer old boy. I would sit with your fiance and let him know how you love and care for him and hope you can work it out and be together and want to provide a stable home for him and your son and will work toward doing that and desire to know if he is up to the task. That is the best any of us can do and every parent deals with things with their children and we need to do it with love, patience and consistency.