Ok, here's what I would do in your shoes:
Suspend contact between DS and STBXH until you have established a safe means of facilitating it.
Stop all contact with your X unless it is via a solicitor.
Send a letter to your X explaining the above, and stating quite clearly that you are not preventing contact you are simply asking for supervised contact while you deal with DS to establish the damage caused by what he has seen. It is a safeguarding issue that you wish to keep low key for now but if you are hassled about it you will involved SS and CAFCASS in a bid to keep things fair. Insist on contact at a contact centre or by a mutually agreeable third party, but not you or his parents.
Tell him that while he is free to email or text you about contact, you will only enter into discussion about the times and dates of contact arrangements, nothing more. Not the validity of those arrangements, which should be discussed via solicitors, not the marriage breakup. Any other communication will not be answered, will be construed as harassment and may result in you involving the police.