Advice please?

Don’t push anything. Wait until they are ready. It does all sound a bit messy, and it could be your son’s girlfriend is just very wary at the moment. Take it she knows best what’s good for her son. You really have no leg to stand on at the moment, but after your grandchild is born, you will then...hopefully, have a better understanding.
Congratulations by the way, and hope all goes well with the birth.

Maybe i am very old fashioned to think
You should meet family and siblings before a new grandchild is born . Seems from replies i am out of touch ( at 48) that scares me

At the end of the day you have no choice but to go along with their wishes. I would back off for a while and hopefully things will cool down once the baby arrives. I would concentrate on establishing a good relationship with your son and his new partner first.

It’s commendable that you want to include this wee boy in your family and I agree with others who say that the dynamics with the ex seem to be the motivating factor here.
That said, he’s only 5 and has had a lot of change in his wee life, a new step dad, a new sibling on the way, a new home and living a large part of his life through the restrictions of a pandemic.
Try and be patient? They seem to have a lot going on settling in as their own family whilst negotiating the pressures of the ex and his extended family.
The ADHD thing may be a genuine concern, but teachers etc will pick up on that when he starts school which means that there will be support for the lad, which the ex won’t be able to argue against without welfare concerns being raised against him. That said my two would never in a month of Sundays sat still to chat to anyone on a video call at that age - never mind after over a year of on and off confinement due to lockdown.
In your shoes I wouldn’t apply any pressure, but follow their lead on this one. Assuming your son and his GF are in this for the long haul you will have many years ahead of you to look forward to with both the children.

Thank you all for the replies . A few points to answer . 1) We are the ones who live 100 miles away . 2) From what we are told the boys grandma is the one who has the boy ( i am led to believe if it was up to the dad he would not bother seeing his son at all , and only pops in when the boy is at the grandmas or has him at his home rarely , the grandma does the school pick up and he sleeps at hers . This was why the girl went to court as she wanted her son not to have to swap every day and half she wanted say 4 days one week 3 another so the boy would be more settled . The grandma was sacked from her job a few years ago for bullying ( we know this to be true as it was reported in the press) 3) We have tried to do facetime , when the lad is there but there always seems to be some excuse ( my son works ling hours and to be honest as they have only just been together a year we have only met her twice , once being at a funeral so do not really know her well enough plus the lad will not sit still or concentrate longer than a minute ( we and son think he has ADHD but the girl is scared that the dad/ grandma will blame her if she tries to find out and take boy off her4) The ex is not violent just mentally controlling by the sounds of it . 5) She had a bad childhood herself from what we have been told her mother was very cruel to her ( she actually ended up in prison and is not allowed in the area ) 6) We just think with our grandchild due soon that is just does not feel right that we have not met its brother . My son is quite easy going and goes along with anything she says ( this is his 1st child and our 1st grandchild and was not planned, but is happy and she and the boy moved in after she told him she was pregnant )

It does sound as if there is more to this around the ex than you know! Facetime seems like a good idea

Can you facetime your son, DIL and little boy? That way he'd get sort of get to know you and you wouldn't be strangers when you do eventually meet.

Ask your son what will be best for them, both now and when your grandchild is born.
Does he expect the situation to change then, so that you will be able to visit?

Back off & leave it be for awhile.
It may help ease all the pressure, best wishes

This sounds a bit iffy to me.

Sounds as if you are only being told part of the story but not sure if there is anything to an do to change things.

There seems to be a battle going on between two warring parties and the real victims will be the children caught in the middle. How is she going to prevent her child mentioning that there is a new baby on the scene! If the ex is so violent seems odd that he got joint custody.

I have a feeling there is something you don’t know.

What is the custody arrangement, why doesn’t the father come to collect the child?
Is the child’s father aggressive?

Do the grandparents who collect the child live a 100 miles away or are you saying you live 100 miles from your son?