It sounds as if some trauma counselling for your child might be a good approach. A psychologist can help them unpack feelings, emotions, help rebuild self-esteem and confidence in the child also if they were ‘put down/belittled.’ If a crime was committed against the child, go make a police report.
If there’s a family court order in place for visitation, you’ll need legal advice to not be in breach of that potentially, if you withhold the child from going to dad’s. Working through mediation might help with that aspect.
If the child is being re-traumatised by visits, I’d advocate for them to not go-at least until the father has agreed to have and actually takes part in some parenting classes/guidance around appropriate behaviours/responses and learns to empathise with the fact that, whether he agrees or not with the child’s claims, he must respect that it’s very real for the child and as a parent, he should want to support them and reassure them that he cares/is willing and able to protect the child from harm going-forward...