I’m not implying either way but I genuinelly wonder what the comments would be if DS19 was actually a DD19. His emotions and feelings are completely valid on the matter. He is allowed to feel however he is feeling and it is up to OP to try to explore that with him and help him to get to a better level of understanding, which I imagine will include addressing some deep rooted worries of his.
Can only speak for myself here, but my response would be the same if it was a DD vs a DS.
I don't agree with other posters about kicking them out or "Just man up" speak etc, but it is absolutely unacceptable imo that a 19 year old is acting like this;
"But , for the past week, he stopped talking to and threatens to never see me or his siblings again if I let my stepdaughter call me mum."
Or this;
"She is William's child , are you not happy with the 3 you have?.... I am going to start calling (Father's wife) mum then"
Or this;
"I am going to be honest, no, he isn't generally nice."
Or this;
"he complains she's "annoying " and "always wants to be with me".
Or this;
"What I dislike more and more about my son is that he is a bit hot and cold. If I don't do what he wants, he gives me the silent treatment. Then I feel guilty and always end up doing what he wants."
Acting like that because a 9 year old girl who doesn't have her bio mum in the picture wants to call you mum - Acting like that when OP has stated repeatedly she's tried many times reassuring him and telling him she'll always be his mum, be there for him, support him.
OP as far as I can tell has done nothing but try to reassure her son, and it's not working, he's still being like this.
Other Poster's have mentioned the brain maturity thing - Yes, the brain is not fully matured until 25 - but at the same time can we please stop pretending that this is normal 19 year old behaviour??
He needs to be reassured, AND it needs to be made clear that this is something he needs to come to terms with and accept whether he likes it or not without giving ultimatums and threats about never seeing the family again.
Poster's have also said, DSD calling you mum has zero actual effect on DS -
Does that mean he should "just man up"? No! But he does definitely need a shift in perspective somehow.
Everything OP has listed off very much gives me the impression he's only thinking of himself right now and is showing little to no empathy for either OP or his SS.
That could be for any number of reasons, fear/anxiety about growing up, narcissism, typical teen "main character syndrome", whatever - Does it mean you should be Kicking them out? No, of course not.
But giving them nothing but support when you don't know what the reasoning is, and essentially reinforcing this frankly quite gross behaviour towards a 9yo?? No, I don't really think that's wise either.
There's a middle ground to be had here, and I feel like it's being missed whilst a few other posters are going back and forth arguing in circles when frankly, everyone has a point to a degree!