I agree with PP. It’s up to your son if he wants to do this, but you don’t need to be involved .
I have some experience of reunions with “ long lost “ relatives and I have to say I don’t think your sons plan is a good one.
He has only met his father once and it sounds like his father didn't even keep up the contact. It would be much wiser just to have another one on one meeting when he is in the Uk and in a neutral setting, like cafe or pub.
It’s far too much to drag everyone else into it - this mans wife, adult kids, you, your son, DIL and baby. Are your son is expected to host all these randomers at his house AND deal with his own newborn baby.
No way.
On top of this, it’s at his house so he can’t get away if it becomes to upsetting. Which it will do.
It will be very hurtful to your son to see how close his father is to his other children . He will wonder why his father hated him but loves his other kids.
He will see his father hold his tiny grandchild and wonder why he was such a bad baby that his father Walked out on him.
It’s a very poor idea to involve his own child - this man has a long history of walking out on people when it no longer suits him - why allow his to do this to the next generation as well?
I know this is your sons plan and not yours and he may not be asking for your opinion . But if it was my son, I’d be suggesting that he meeting his father alone for lunch NOT in his home. No one else involved.
You say that your bear no animosity to your son’s father for abandoning you both when you were pregnant and having nothing to do with his son for 28 years . This seems remarkable to me - don’t you believe that parents have a legal and moral duty to care for their children? Do you come from a culture where many children are brought up In state care ? It seems very odd to me.