Were you married to the child's other parent? Who is "they?" Assuming you have a parenting agreement, look to its provisions and attempt by conversation to remedy the situation. If this is not possible, you may need to return to mediation and/or court. Your question is a little light on facts but you probably should return to your attorney, or consult with a new one, to discuss the best way to handle the situation. There is never an absolute cure for imperfect parental conduct.
This is your second (at least) post. You need to take action; venting online won't accomplish anything and won't help your son.
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How do you know your son is being interrogated? Be very careful in trying to obtain this information directly from your son. You don't want to put him in the middle any more than what your ex-husband is. Also, be careful engaging your ex-husband in emails - sometimes this form of communication conveys something different than what is intended.
The real question is what do you hope to achieve? If your goal is forcing your ex-husband to stop communicating with you in this way, this will be difficult, expensive, and probably unsatisfying in the end. If your goal is to change custody, then this type of evidence can be used to help prove your case.
Nothing in this post creates an attorney-client relationship. This post is not meant to serve as legal advice.