"No, they shared my son's info with my partner's ex. I have nothing to do with his
child support to the ex"
So Im assuming your partner is also dad of the disabled child? So you guys provided CSA with info? and your partner pays child support to his ex?
So for example you're partner is seeking to reduce his child support to his ex on the grounds that his son (to you) had special needs. Then CSA would consider that and may reduce the child support payable to his ex... Now that decision impacts on the ex and as such she is entitled to the info regarding the reasons for the decision. If she didn't have that info how could she appeal it if she felt it was wrong. So there is lots of transparency here.... BTW I kind of hate it...
I have 3 kids living with me... I do my tax every year and every year my ex get to find out how much I earn. She doesn't do her tax - claims an income of $0 - so I kinds see your grudge. BUT the transparency is necessary - and sad when it is abused....
But the problem is the partner's ex using info / photos and you asking her not too.... Now you could spend a bucket load of cash on a court case - and between you and me I'm not even sure on what grounds - but lets say you go see a solicitor and he makes a case and you win... I can't imagine how much money that would cost and how much stress. Is it really worth it.
Now as far as Facebook goes - a kid sharing a photo with her step-brother could be seen as cool - I'm a father in a blended family and we post photos of my my kids and step daughter and don't see anything wrong with it - Modern living.... Oh and how is that photo damaging your kid?
Short version - you gotta pick your battles and sometimes the best thing to do is to pretend you saw nothing and no nothing because asking your partner's ex to remove photo's or not share info is polite - but if the ex is gonna enjoy knowing it is upsetting you then all you've done is give the ex the loaded gun and invited them to aim it at you (if you know what I mean).