Calling somebody else dad

Not good.
My family member has been with his partner for 4 years and lived with her for almost 1. Partners DD has known my family member from 8 months and has never met her biological father. DD still calls him by his first name, her Mother has never suggested differently.
The title of Daddy must have come from your brothers ex. I feel sorry for your brother as it's going to be very difficult to change now.

A name does not validate or define a relationship - it's who you are and how you behave, not what your name is.

I agree with that completely. I also think children often try to figure out new relationships by testing out how they could match up to what's already known. A child calling a step-dad 'daddy' is probably trying to understand how to parse that relationship. Both parents ought to be helping the child and talking more openly about what has happened.

Oh yes! I certainly feel like that sometimes but honestly the best course of action was 'training myself not to care'

My mum married my sf when I was 5. I started calling him by his name, then cycled through various different options, father, step father, daddy, dad before settling on Dad. It was the option that felt right to me.
It was absolutely driven by me, not by my dp. Don't assume that this is being driven by your nephews mum and partner.

Just encourage your db to be a great dad, listen to his ds, be there for important events, school plays, sports events and training, hobbies. That's what builds the bond, not the name.

As his auntie, I'd be gently telling nephew that Stuart isn't his dad and he shouldn't call him dad. Maybe think of another nickname for Stuart together. It's really irresponsible/hurtful of his mum to do this when his dad is in the picture. I'm not surprised he's upset. This of course is assuming that your brother is very involved with his son and not flakey or unreliable.

Wow I'm surprised at the responses here actually.
I entered a new relationship when my DS was three. My exDH and I were always civil and friendly and DS went to see his Father often. My new DP and I then had a DD together. When I was pregnant, my DS asked if, when the baby was born, my DP would be the Dad of the house. I said he would as he was the new baby's Daddy. He seemed happy with this explanation. Once our DD was born, my DS just evolved into calling my DP (now my DH) Dad. He'd talk to our DD and say 'oh look at Daddy' etc etc and it just evolved. We never told him he had to call him Dad, he prior to this, always call him by his first name.
DS always knew who his father was, he adored him and still does and continues to have a wonderful relationship with him. DS was just lucky enough to have lots of people to love him. Also, just so the trolls don't have a go at me asking me how I'd feel - when my ex DH married again, him and his new DP had a baby and my DS then called his father's new wife Mum, again due to the evolution of the relationship and the new baby coming along. But again, my DS knows who his mother is.
We (My exDH and now DH) always wanted my DS to have a loving family and always wanted to avoid us and exDH having a toxic relationship. We ALWAYS put our DS at the centre of all the decisions - he was and is the most important person in the dynamic and his needs came first. A name does not validate or define a relationship - it's who you are and how you behave, not what your name is.

@dinku5 thats great that you are now ok with it but it makes you sound like they see you as a handmaid that has provided them a child and were just an incubator.

It's not ideal as he hasn't been in his life long BUT if this comes from your nephew wanting to call him dad opposed to being told to then thats a bit different.

At the end of the day the more people to love children and to be there for them providing a safe and happy home the better.

And while I wouldn't make a big deal of it or would try not, if I heard my child saying that I'd say 'no I'm your mum not her'. Because it's right. That's a fact.

Well I disagree with other people being called mum or dad.
Possibly a bit more understandable if there's no contact at all.
Not calling the step person mum or dad doesn't mean they can't have a close relationship for god sake

I honestly think it's bollocks to imagine this has anything to do with putting a child first or making them feel comfortable.

I can understand dad being upset. He is the dad, not Stewart. I think mum is being very stupid and irresponsible. She hasn't know Stewart for 5 minutes and she's moved him in, is pregnant and letting her son call him dad. There's not a huge amount dad can do about it. How often is he seeing his boy? He needs to up his contact and make sure he is present and a stabilising factor in the child's life.

My child calls their step mum 'mum' also 6 years old. Burnt like hell at first, I hate the phrase 'tummy mummy' even more which is The explanation dad and step mum have given the child as to who I am. I have majority contact. Now, it doesn't really bother me. It doesn't affect my position in my child's life or our relationship.

Moving a man in and getting pregnant within the space of a year is not "putting the child first".

It's understandable the bio dad is upset.
However, I'd advise him to not make a big deal of it, as the boy is so young and probably just trying to make sense of the situation.

He needs to just keep on being a good dad, and his son will grow up knowing he is loved.

Being a parent is a thankless task.

In my experience adults who make this argument are usually pretty good at signalling to children what they are supposed to "want".

This is emotionally abusive and confusing for the child. He's not his dad.

The only time I think this can be ok is if bio parent is completely absent or has died, and after a long time.
Eg DHs bio dad has never met him he was physically abusive to MIL (she had to flee to refuge pregnant) and never wanted anything to do with DH. When DH was two MIL met FIL (DHs step dad) , he remembers clearly calling him by his name, and when he was about 8/9 asked if he could call FIL dad, by this point they were married and all lived together. To this day he calls him dad and I refer to him at my FIL there is no step about it, he had absolutely been a dad to DH in every way. He's a lovely man.

This isn't the case here those isn't even step dad this is mum's boyfriend and dad is very much in the picture.

In his shoes I'd be devastated

Of course not being unreasonable. The poor dad must be gutted - anyone with a basic sense of decency would immediately say 'yes, I'll have a conversation with DS and you should too'.

It's fine - sometimes even beneficial - if a child feel s/he has a parental relationship with adults who have relatively newly stepped into that parental role. But that doesn't mean it's ok for the dad who has been there since the start, who is still playing a parental role, to be ignored. Some of the responsibility for helping the child work out it's ok to say 'this is my dad ... and this is my step-dad and my mum' has to rest with the mum here.

Why? Is it not best to put the child first and make sure they feel comfortable in the situation.

At 6 years old, it's likely his mum is encouraging him to do it, not the child's decision.

This is so awful

I have a niece who calls her mums new partner dad, it’s a bit annoying when she talks about dad and we know it’s not her bio dad that she is talking about.

Not unreasonable to be upset but for me it’s down the child what they want to call people in their lives. Perhaps he could have a Dad and a Daddy or DadStuart and DadBob.