You can’t and neither should you stop contact. I know it’s hard, and I know that there is a thought process that children should be able to make up their own minds, and to an extent this is true. But at 12 a child really isn’t old enough or capable of knowing what the potential consequences of never seeing their other parent again are.
My own ds essentially stopped going round to his dad’s at 14, but it was a gradual thing, and before that I had very much encouraged him to see eXH, because whatever I think of him, he is DS’ other parent, and it is not up to me to have any influence over that what so ever. He was 14 almost 15 when he stopped staying overnight at his dad’s, and now while he doesn’t see him often he has a better relationship with him than I’m sure he would have if I’d told him he never had to see his dad again when he was 12.
Also this sentence rings alarm bells for me:
I have moved on and am engaged to my soulmate and pregnant. DS is thrilled he was asking for a sibling since shortly after meeting my DP. DP has been a wonderful influence on DS. DS has ASD and his social and communication skills have blossomed, his motor skills have also hugely improved because they practice catching, throwing etc.
Clearly you’re happy with your DP, but it seems very clear that you’re wanting to paint you, your DS and your DP as this little family, to the extent you say that your DS asked for this sibling, and to be absolutely honest, his having a relationship with his father gets in the way of that. It’s great that your DP has a good relationship with your DS, but he’s not and never will be his father.
it’s fairly obvious here that you are influencing your DS’ thinking, even if subconsciously. You need to take a step back from the wanting to think about your DP as a replacement for his father, and keep encouraging your DS to see his dad.
While the courts will take a 12 year old’s feelings into account, if it’s obvious that you’re influencing your DS like this the courts will grant your ex access, maybe not 50/50, but there’s no way of actually knowing that.