To let my step daughter call me "mum" ?

This. And his emotional development has been stunted because of it, in spite of his mum's best efforts. He's struggling and handling it all terribly. All OP can do is show him this and point him in the right direction of working his feelings through in therapy, with or without her.

Telling him he's a selfish, coercive little shit is unlikely to get him to see things differently, but more likely confirm his feelings that he has no sacrosanct position in his mum's life.

Hearing, feeling and knowing are all different things.

Good post by @Calliopespa 00:31 nails it. Succinct & clear post

Because at some point in his developmental journey his feelings of security have been damaged. That title is emblematic of that for him. Secure people don’t feel jealous. He has felt replaced and no one should have to feel replaced in their own parental dynamic. I don’t think anyone isn’t feeling for the little girl. We are just saying his feelings need to be addressed, not ignored for her sake ( which ironically would actually confirm his fears) and it’s OP’s role to ensure he gets help with that.

Take a car trip with son and talk, visit a coffee shop. Remember to give him alone time with Mum.

Suggest that if your son wishes to call his stepmother 'Mum" then he is at liberty to do that. He can call two important women in his life Mum without changing the fact that you are, and always will be, his mother and will always have the special mother - child bond.

Try to get him to walk in his SS shoes - a neglectful biomother whom she doesn't know, all her friends at school calling someone Mum, all the other kids in her house calling someone Mum. She just wants to feel normal. It doesn't change the fact that she can never be a biological daughter or sister.

She is a child and your family is her only protective family. She is here to stay and will never usurp him. You will all be as kind and caring as you can. Accompany son to a counselling session if he still can not comprehend.

His comment is reasonable about calling you Mum Op(name) whenever she feels like it.. That is a really good way of her always having two options particularly if she is ever back seeing her mother. Compliment son on that idea but make it clear that she can use either - it's her choice.

Yes, because his feelings are completely and utterly unreasonable. You can't even give a good reason for it.

If you can't work out a reason for why he might possibly feel insecure and vulnerable you're being really really slow. Best not to say too much then.

Have you considered having counseling for yourself and your son to talk this out?

Yes, because his feelings are completely and utterly unreasonable. You can't even give a good reason for it.

@slore have you actually read the thread
the posts about his anxiety, fear,and irrational feelings. He perceives he is under threat the very person he loves(his mum) may be less available, or he may be excluded or marginalised. He’s a scared young man acting out. Because he feels the 9yo is taking his mum away or destabilising the mother - son relationship. It’s not logical it fraught and emotional and not in his conscious control

I'm of the opinion his behaviour is unacceptable. Maybe it's coming from anxiety but he's an adult and she's a young child he should not have a childish tantrum and try to control you via threats. He doesn't own you

I do think it's his anger coming out maybe toward your DH and yourself regarding how he felt to have a new step-dad in his life etc. Maybe he sees that she will have two people she calls mum and dad and have more of a stable upbringing than he did at her age.
I also think her being female is an important part of her wanting to call you mum. She obviously already seeing you as her mother figure and I am sure her role model.
Are there unresolved issues around your son and you and your husband? It sounds like he had a lot of change when we was younger and maybe he is feeling displaced and like you see yourselves as a family without him.

Well he clearly thinks it does. Should his feelings be dismissed??

I actually despair of people, reading this thread!

Look, he’s 19, he’s moving into a different stage of life as an adult, different relationships with his parents and step-parents. It’s completely natural that he feels as though he is losing his place in a family which is, inevitably, dominated by the three much younger children. He doesn’t need telling to grow up and he doesn’t need to have his feelings dismissed. Listen to him, be kind to him, and give him some time to deal with this. Being a kid of any age with a blended family is hard and he’s allowed to feel that way.

And how does his 9 year old stepsister - who has known his mum as her sole mother figure from age 5 - changing to calling her "Mum" take anything away from him at all?

I couldn't care less about the basis for you using the term, because it's just so much bull!!

This is a hurt teenager lashing out. Get real.

That is completely unfair.

The OP says Overall, I have never seen him behaving rudely towards her and that he gets on great with the twin siblings.

He is just insecure about his relationship with his mother, and/or his place in the family.

And what about a 19 year old who still needs his mum too?

You haven't got a clue, have you?

I see vile✅ and bellend✅are trending in the get ‘im told school of parenting
All online bluster and blag of course. Online getting all aerated and Phil Mitchell

FWIW when I used the term ‘coercive control’ I was using it to describe his behaviour towards his mother in context with the dictionary definition not in reference to various psychological definitions describing intimate partner violence.

coercive control
noun [ U ]UK /kəʊˌɜː.sɪv kənˈtrəʊl/ US /koʊˌɝː.sɪv kənˈtroʊl/
Add to word list

control of another person's behaviourby using force or threats, or by causing* fear:*

It's not about your son. This is none of his business and nothing to do with him, his situation is different than his stepsister's.

Also, he is an ADULT and should not be getting jealous of a nine year old who wants a mum!

That bit got me, too. A nineteen year old always being her top priority, regardless of a nine year old and two four year olds? The word "manbaby" comes to mind!

This.

Imagine behaving like that because a little girl wants to call somebody mum, who has very much fulfilled the role, after not being able to say it for years, if ever.

He needs to get a grip he's being absolutely vile.

But the truth is, he has been replaced.
For 12 years the family unit was him and his mum.
Then his mum joined William's family. So now the main family unit is his mum, William, William's daughter, their twins. And he's the odd one out.

Ten pages and that’s your considered opinion? Cannot see any reasons for this behaviour. No coherent suggestions regard satisfactory resolution

Catch an actual grip!!