In your position OP all you can really do is attempt to reason with your ex. There probably isn't much hope he'll be amenable, given what a disgusting thing he has done to his wife and children, but you can only attempt it.
If he surprises you and is open to it start by working out a set contact schedule, so your DS knows exactly when he's seeing his dad. Suggest that he builds his own relationship with DS back up for a couple of months before introducing the partner.
Then she can begin joining for brief meetings, before your DS begins to spend his time with his dad with both of them. Hopefully he will get used to this set up, of going to his dads on set days and seeing his dad and the partner, before his new sibling arrives.
If your ex, as he sees likely to, ignores your request then all you can really do is support you DS. If it comes to it and your ex won't even agree to a set schedule you might be best to consider court. This obviously comes with the risk of him asking for more time with the children, but given you yourself have a newborn and he is busy creating a do over family he and his partner probably won't want the hassle of having your two children much of the time.
The reason court might be beneficial if because set days would then be pit on place. He can't just swap amd change as and when he chooses. He can decide not to show up or take the children on his days, but this would serve as good evidence of his lack of commitment if he ever went for more in future. Then if he did stick to it it would mean you know where you stand, and what your responsibilities are with regards to sharing holidays and making the children available to him.
He isn't putting his children first. Even his unborn baby is going to be born into this chaos and to a father who is moving about between women and not taking into consideration any of his children's needs. All you can do is minimise the damage the best you can. Thankfully your children have you to hopefully compensate for their fathers inadequacy.