Trouble child. Co parenting, HELP!!!!!!!

My child’s father and I have been separated since he was two due to domestic violence while we were together he was abusive. My son is now kindergarten and when he’s with his father is when the behavior starts. When he’s with me he’s fine. The school has notified me they can tell when he’s at my house vs when he’s at his dad. I know when I was younger I had episodes every one in a while vs my son it’s every week as to we split the weeks in half 4 days on 3 off than 3 days on and 4 days off. I’m going to sit down with the dean this week to get him on a track path. Any other suggestions? As to if I tell his dad my fiancé will handle it I know he would be pissed but my fiancé sees how stressed I am and helps with him but doesn’t want to over his boundaries with my sons father. I can’t have my child labeled any more or have him suspended so close to the end of the school year. My child is very intelligent and is ahead of his class. Is he not challenged enough?