Nightmares

Melissa • Mummy to two coming 3 (due August) beautiful children.
My son is 4 years old and his dad was very abusive to him, but with lack of evidence in court and him being so young and not speaking out himself about It, his dad's mum got contact one day a week overnight. But she is trying to manipulate my son into wanting to live with her. But he has voiced to me that he always has nightmares when he stays over. Why would he be having nightmares?