I’m also in Australia. In the 5 schools my children have attended (4dc primaries and high schools), when there are separated/divorced parents, and each parent is registered for the school app/newsletter/school photographs etc, for things like graduations, concerts, recitals, each parent has an allocation of tickets, which is why there are often 4 (or even 8) available, and it is absolutely nothing to do with one parent what the other parent does with their allocation. So the mother in this situation would have zero say in who uses the father’s allocation of tickets.
I am also a step parent of two grown dc, and my late husband and his ex wife were almost no contact, and never in a million years would DH have attended any school events with his ex wife, far less a dinner afterwards (the kids wouldn’t have wanted them to, to be fair). Everyone attended, and everything was amicable, but he would never have entertained being dictated to by their mother as to who could attend or that he shouldn’t speak to his dc about it. His eldest lived with us for 9yrs (went to his mother’s every second weekend), and wanted me at everything, if he hadn’t I wouldn’t have even considered going, but if DH’s ex had demanded that I didn’t go, and he wasn’t allowed to discuss it with dc, frankly he’d have told her where to go.
OP, I think you and DH are taking the sensible and correct response to the situation, you’ve said you’ll only attend if DSS wants you there - fwiw, I have a sneaking suspicion that you’ll find he does want you to attend, it sounds to me like the ex is the one trying to manipulate the situation without DSS’s knowledge, and he might actually be quite hurt if you, for seemingly no reason, didn’t show up to his graduation. Who knows, maybe that’s the ex’s goal in all of this, to make it look like you just didn’t go because you don’t care - after all, if DH doesn’t discuss it with him beforehand, then as far as DSS knows, you are just a no show, with no explanation.
And if he doesn’t want you there, because his mum might make a scene, then although it will be hurtful for you not to see him graduate (and yes, high school graduation is a big thing here), at least you will know that he is the one who has made the decision, and you can celebrate later, as obviously from your OP you and DSS have a lovely relationship.
Finally, if, as I suspect, your DH would rather eat dirt than attend a cosy dinner for 3 with DSS and his ex (and I wouldn’t blame him), then he can gently explain to DSS that although he is happy to attend his ceremony, it wouldn’t be comfortable (for anyone) or appropriate for the dinner to happen in that way.