spidookly, yes he did steal from her the right to tell her ds some devastating, life changing news. As his mother, and with his best interests at heart, she didn't rush into telling him becasue she wanted to digest the news and try and find the best words and situation possible in which to gently break the news. That moment, she was well aware, would have a huge impact on him, and it's one he#d be likely to remember for the rest of his life.
So, knowing she was not rushing into to, dp, for whatever reason, impatient, thoughtless, cruel, undermining, whatever his motivation, he decided to take away her right to tell him, and his right to hear it in a loving way from a loving person.
I don't think any parent on here , if faced with telling their dc that their other parent had died, would think any old person should be able to tell their dc any old way they wanted. Of course it is their right. It is not 'imagined'.
r ( we don't know do we, whether or not they have been together a couple of weeks or a couple of years, or whether or not they live together, how much the dp is involved with the ds and how close they are).I agree, I don't think splitting up from partner at this time is necessarily ideal, but him telling the poor boy in this way could well have caused him huge problems, and he could look back on this and resent the mum for staying with the dp and see it as mum not protecting him.
I dunno, thank God I have never had to experience such a situation and hope I never will, and I wish the OP and her ds all the strength and wisdom she needs to cope with the coming days and months.