If the OW ever calls you, hang up without saying a word. You are under no obligation to speak to her. If she comes to your home, don't answer the door. If she contacts you elsewhere, file a restraining order.
Keep a log of every call, starting today, so that if you do need to seek a restraining order, your evidence is handily available.
I would not change the DS' name for some time just out of spite, but that's me. :) I still think you shouldn't do so until a few years down the road, when you have some distance, and, therefore, more perspective on the issue.
I believe the fact that the ex has to meet his financial obligations is what triggered that call. If you texted him, and she saw it, it is because he showed her the text. I suspect it was because the ex wants OW to intimidate you, he doesn't want to meet his financial obligations to his son. He believes this will work, and to some extent, it has, as you are crying. That is what they want. They know that his requirement to meet his legal obligations to your son will crimp their lifestyle. All of a sudden, those vacations may not be as frequent. OW is a bully, and thinks she will make you surrender if she harasses you. Your ex is weak, allowing OW to fight his battles for him. So, Monday morning, start your claim and let CMS deal with the child support.
I would do one more thing. Send him a politely worded email with a read receipt notification:
Dear X,
You have told me you are too busy to spend time with [Son's Name]. I encourage you to see [Son's Name], however, I will not push it if this is something you do not desire. Please call or text me directly when you wish to see our son.
I would do this so that there is a recorded message that you did not block access. I suspect he will try to use his non interest in your son against you at some point, possibly to slink out of his financial obligations.
After you write the message, print it and save it, and don't delete it. It will prove that at no time did you restrict his access to your son.