It is hard yes, but he is your dc's family and his partner and her children are part of his life now - your dc has a right to be involved in his life, to feel loved and cared for by their other parent.
My dp's ex wouldn't let the dc's meet me (by wouldn't let I mean she made her feeling perfectly clear on the matter, they were teens and could make their own decisions but at the end of the day they live with their mum, have to go home to the house they share with her and by all accounts, not just dp, she is a difficult woman).
Nearly 5 years down the line and I've met one a handful of times, which I don't think his ex knows about, and have just met the other, who has been away from home as a student for 3 years, for the first time (and only because of a recent health diagnosis for dp).
We are meant to be getting married soon and both of his dc's had said they will come but neither want his ex to know, and tbh I'm not sure they would come anyway.
My dc's have never met his dc's.
This situation is hard for and hurts dp, but I suspect it isn't doing his children any favours at all either, and they are left with dealing with their mum and dad separately, not being able to mention their dad to their mum, not being able to visit their dad's home, not being able to be a part of their dad's life.
Yes he could have gone to court, and probably should have done, if only to show his dc's that he would fight for them as given their ages I don't think court would have achieved anything.
As it was he took the quiet, being there for them as much as he could, supporting them, view - that they were better off with him in their lives to the extent he could be rather than having the conflict of court.
I have no idea which would have been better - I do know that a far preferable option would have been not to have the issues in the first place.
My ex left, probably for his current partner although I don't know that for a fact, and our dc's have lived with him 2 days a week from the moment we split (2 and 5 at the time).
He gradually introduced his partner and her child, then they moved in together. My dc refer to their sibling when talking about her.
It's very hard for you, but please make it as easy for your dc as you can.