Not allowing father to pickup child for visitation

My husbands EX says that she will not leave their 9 yr old son be picked up on the next scheduled visitation. There is a court order in place and we know that she will be in contempt if she does not allow him to come with us, however what do you do when it happens? Do we call the police? How do you prove that she is in contempt? HELP!!!!!!!
Also,
The reason for her saying she won't let him come over again is because we questioned him about a bruise that he has on his neck. When we noticed the bruise and asked him about it he immediately started to cry. This prompted us to be suspicious of how it got there. He kept telling us he didn't know and then said his brother bit him, then he changed the story to say he was wrestling with his mothers live-in boyfriend and maybe that is how that happened. We explained to him that if someone is hurting him whether it be the mother, the boyfriend or someone else he needs to tell us so that we can help him. I guess I came on a little strong - questioning him, but I am so worried about him. (When he was 5 his mother had grabbed him so hard that he had bruises on his arm for days. We took him to our local Children and Youth services, he told them exactly what happened. They investigated it and then said it was unfounded). She used to have another live-in boyfriend who the son said used to hit him. We never actually saw bruises so there was no way to prove it. And there has been other claims of abuse which have promted us to look for these signs of abuse. Anyway, back to this incident, she claims that he told her that I said she choked him. Which I never actually accused anyone. The strange thing is that we were in the house no longer than 10 minutes after dropping him off - and she was calling to yell at us and to say that he is terrified of me. It sounds to us like she knows what happened or did it to him and questioned him as soon as he got in the door as to whether we saw the mark or not. I don't believe that the son would have walked in the house and said that I accused his mother of choking him, when that wasn't what actually happened on our end. She said she is calling children and Youth services on me because I am mentally abusing him. Another thing she said is that he says that I pick on him because I make him brush his teeth and clean his room. Any Advise on how to handle this situation. We are afraid she won't let him come over. And what do we do if that does play out?

I DONT HAVE MUCH LEGAL ADVICE. I DO KNOW THAT IF YOU CALL THE POLICE,AND CAN PROVE THE CHILD IS SUPPOSED TO BE WITH YOU,THEY WILL EITHER BRING HIM TO YOU , OR FILE A REPORT AGINST THE MOTHER FOR INTERFERING WITH VISITATION.
THINK HARD BEFORE DOING THIS.IS ONE DAY OF MISSED VISITATION WORTH THIS CHILD SEEING YOU THE STEP MOM SEND POLICE TO MOMS HOUSE?SOUNDS LIKE HES HAD LOTS TO DEAL WITH ALREADY.IT SOUNDS TO ME LIKE THERE IS A LOT OF BITTERNESS BETWEEN THE ADULTS INVOLVED,AND THE CHILD IS GETTING STUCK IN THE MIDDLE.IF AFTER PUTTING YOUR FEELINGS FOR MOM ASIDE , YOU TRULY BELIEVE THERE IS ABUSE , TAKE ACTION. BUT CONSIDER THIS- CHILDREN GET BRUISES, AND CHILDREN DONT ALWAYS KNOW HOW TO SAY EXACTLY WHAT THEY MEAN.
MY SON WAS AT ONE POINT QUESTIONED AT THE DR. S ABOUT A BRIUSE ON HIS FACE, BECAUSE HE SAID "MOM DID IT"(I HAD OPENED THE CABINET WHICH A CAN OF BEANS FELL OUT OF THAT LANDED RIGHT ON HIS HEAD.)OBVIOSLY NOT INTENTIONAL, BUT IN HIS EYES- I DID IT.BE CAREFUL WITH THE QUESTIONING. YOUR INTENTIONS ARE GOOD, BUT ITS A HUGE AMOUNT OF PRESSURE ON A CHILD, AND THEY WILL ALWAYS BE INCLINED TO SAY WHAT DAD WANTS TO HEAR AT HIS HOUSE , AND WHAT MOM WANTS TO HEAR AT HER HOUSE.IS THERE ANY POSSIBILITY THAT THE FAMILIES COULD COME TO A MUTUAL AGREEMENT TO STAY OUT OF EACHOTHERS HOME LIVES, BUT WHEN THERE IS A CONCERN THE ADULTS SPEAK ABOUT IT AND LEAVE THE CHILD OUT OF IT? WITH ALL OF THE CONFUSION HE MAY GET TO THE POINT WHER HE WONT CONFIDE IN ANY OF YOU.