You were an (insert mamono) who was raising your nephew after your husband's sister had died. He was nice but his father was completely out of the picture for most of his life since he was 5 years old.
It didn't bother your nephew too much and he was happy with you and your husband and didn't miss him at all.
After almost 12 years he returns and wants to be part of your nephew's life. Your nephew is really excited and forgives him and they even plan for him to go live with the dad. However you eventually find out something that made you very upset.
The dad was planning on walking out of his life again and only came back because he needed a place to hide from some people for a few days. He was planning on lying to your nephew again to get out of it. You know it will break your nephew's heart to tell him that his dad is leaving again and never really planned to be part of his life, but you also think he should know the truth.
Potential course of action: Well as a mindflayer I can solve this with a simple lobotomy style procedure. He cant leave if he cant remember where was going to leave to or that he even wanted to leave in the first place.