I can see how you feel triggered and upset by this - all of it is very raw and brings back a lot of emotion. But it is not your responsibility to manage how your ex (because he is your ex) tells his son about the baby. It is really not. I understand you are coming at this with good intentions, but you are making something your issue when it is not.
Your responsibility is to be stable and secure for your son, to say yes, his dad has told you too, and carry on as normal, talk if your son wants to talk etc.
If it helps, when my DD found out about her half-sibling, her sibling was already two. I suggested to her dad (once I got over my annoyance that DD (and I) had not been told) that he start by showing DD a photo when it was their time together and explaining her name and that it was her sister by her dad. That is what he did. We had been separated a year at this point. I did not see it as my role to be there too.
I would be worried both parents sharing news might create the impression of togetherness and confuse your DS who has got used to you apart?
Anyway, these are just my views. I just explain my experience so you can see where I am coming from, not because I think you should follow my advice. You know your son best. If you provide a stable, loving and secure environment, which it sounds like you do, your son will be just fine.