After the custody battle for my boys with my ex I signed up for a website called Our Family Wizard (recommended by lawyers/judges) since my ex was very demeaning and abusive in his text messages to me, even during the custody situation. It has helped tremendously. It allows us to leave messages to each other updating the other on our boys, etc. It documents everything just in case anything were to happen. I can also post medical bills and any other out of pocket costs that insurance does not cover. He can even pay me online through this system so we do not have to exchange anything via our boys. It costs money but to me it was worth it. Any time he sends an inappropriate text or email I write back, "please communicate through the website".
Regarding what your STBXAH said to your son, that was just dirty. It sounds like he is just trying to get back at you through your son.