Things have been quiet between DH and his ex for a few months; the court order left very little room for manoeuvre so DH has been picking DSS up and dropping him off as his ex required - usually to DSS grandmas house, which suits DH as he doesn't have to see his ex. All arrangements are made by email.
But, in the last few weeks, DHs ex has referred a few times in her emails to a particular document she wants DH to sign (relating to Investments in DSS name, on which they are both signatories). DH made it clear several years ago that he's not prepared to agree to what she wants, but every year, she drags it up again, demands he does what she wants, and throws a hissy fit when he won't. Shes cited his refusal to agree with her on this matter as "abusive" on court documents and statements.
Ex has now emailed DH to say that she has rearranged her plans for the weekend so that DH can pick DSS up from her rather then grandma as previously agreed, and that DH can "sign the forms at the same time".
Based on past experience, DH knows that if he doesn't do what she wants, she'll cause a scene - lose her temper, shout at him, drag DSS into it and it's likely to result in DH leaving (to avoid her abuse) without DSS. If that happens, then it's unlikely that contact will resume. DSS is 11, and old enough to decide whether he wants to see his Dad or not - and he only ever wants to see his dad if his mum is ok with it.
DH feels backed into a corner. If he emails his ex back and says he doesn't agree and won't sign the form then it's inevitable that ex will bad mouth DH to DSS, and DSS will decide he doesn't want to see DH at the weekend, but if DH leaves it and doesn't respond, DSS will see his mum having a go at his dad, get upset and decide to stay with his mum, anyway.