My son wants to speak to his dad

My 14 year old son has had a hard time adjusting to our move. He has autism. We moved because my ex husband who is my son's dad, 17 year old daughter's step dad was sexually abusing her and then she was being bullied and my step daughter made the bullying worse by convincing everyone she was a liar. My daughter wanted to kill herself. My daughter has taken well to the move and made friends. My son has not. He had to move away from his friends. We live somewhat close to train tracks so the sound overstimulates him. He's not used to the new house so he won't get up at night to pee and will just pee in the bed. Then his sensory issues go off in the morning because of the wet feeling from peeing in the bed. He's stressed at school and has had break downs from being overstimulated and not having any friends and he went to hitting himself. He asked me today if he can talk to his dad. He knows why we moved. He knows what his dad did. His dad got 40 years after he plead guilty. He knows. He's having a hard time coming to terms and accepting everything. I told him no and that he knows why we aren't talking to his dad. So he asked if he could at least say goodbye. I said IDK know about that and he got mad and said everything is different. He will never see his dad again. He has no friends. He hates it here and he wants to go home and can't even say goodbye and started hitting himself. I had to get him to stop and kind of hold him into a hug until he calmed down. I'm not sure what the right answer is. Especially since his dad has sent my ex step daughter to say he was forced into the plea deal and my daughter is a liar. I feel he would use that last conversation to villianize my daughter. What's the right thing to do?

@Britney yes. My son and daughter have different dads. My ex husband was my son's dad and he was abusing her.