who cares what she thinks or what anyone thinks. What is important is you and your boy and your family - what anyone else thinks is irrelevant. I understand social services and the police are involved but you said upthread that you have many years parenting your boy together. why do you want him involved with your boy? I understand your boy wants to see his dad - but the fact is his dad doesn't want to see him. That is clear.
You sent the letters recorded delivery, they were rejected (or kept and not handed over). You've done your bit, you don't need to do any more, the evidence is there that you did your bit.
Chasing and forcing him to engage with your boy is keeping you intensely engaged with ex/them, heart and mind. You have to disengage, you know this is the only way to peace of mind and sanity. Yes the waters are muddied bcs of your boy's SN but your boy needs consistency and he is not getting that with his dad. I also very much doubt ex is keeping your boy away from OW.
It's all grotesquely unfair and wrong and heinous - but that's the way it is. Accepting that is the way forward, not just for you but for your family re you will be very distracted, unavailable, if your focus is on the hideous pair, what they think, what they're doing and saying. Youccan't force justice Mrs xx