Need advice

Little bit of Back ground information. I am co-parenting and agreed to joint custody. My ex has been mad for 6 years I've been seeing someone else. Judge agreed that he should have access to the kid every other weekend and every Friday.

Out of the blue my kiddo who is 7 stopped wanting to call his bio dad dad and only wants to call my boyfriend that I've lived with dad.. I've tried talking to my kiddo about this and my kid is refusing. We just started therapy but literally just started it. He keeps trying to run home from school and from his bio dad's house, the kiddo is hitting and having accidents at school. His bio dad is mad at me cuz of the behavior but bio dad refusing to cooperate with therapy for our kid and is always mad. I could use kind words or advice if anybody has ever experienced something like this any advice. I only have this one kid so I'm still learning.