OP, in this situation, understanding how you feel is not the same as giving you practical advice.
Of course the thought of your ds never seeing your family if you were no longer here is hard for you to think about. But it's one of those things that no matter how much you worry about it, you have no real control over. And it's not worth worrying about things that may never happen.
So what you need to focus on is what you do have control over. You may be able to ensure that some of your money goes to your parents. You can certainly express your wishes in your will, and hope that those will be respected in the event of your death.
You also need to remember that you are thinking of your ds as he is now, and presumably this will will be in effect for a number of years. By which time, your ds could be doing exams, and your dh would have a better realtionship with him so would be better placed to see what is best for him. Even if your dh is selfish, he could still decide your ds needs to stay put while he finishes education. He may decide to move and it could be a great thing for your ds. Wherever he is he would have to deal with life without you, but maybe it wouldn't be as bad as you think for him in NZ.
FWIW, I do understand, I have will issues of my own, and I worry about how much my mum would get to see my dc if they went to their Dad (who is a great Dad, but we're not together) But I do think that when someone dies, things change dramatically, differences get put aside when people realise what is most important. Before the tragedy happens, you can't predict what will happen with any accuracy at all, you really can't.