When you met him I assume he was still married and not divorced?
When was the divorce finalised because I think that is Day1 and recovery starts from then.
Please do listen to others and don't just believe his narrative. He stayed with her so she must have many redeeming features. Don't fall into thinking he is a good and she isn't. If he has held down a demanding career he won't be a victim or walk over.
Ime when they there is an issue with dc and a non resident parent then I suspect that your bf has played a signifcant part in that relationship detoriation. I only know this because my dc do not want to meet their dads gf, absolutely nothing to do with me but they just have zero interest.
The reasons are complex but mostly because he doesn't have a good relationship with them and they don't feel comfortable with him, let alone the dynamics of a gf. He has tried to push her onto events and even holidays but they would prefer to not see him if he pushes his agenda.
I know Ex will rather blame me than accept or admit that he doesn't have a good relationship with his dc. The relationship he has is superficial rather than a real connection..but they also think he can act like an arse. If his dc are adult, trust that they know him better than you.
I have a friend who's now husband has lost all contact with his adult children and only now after 10 years can he admit that he behaved badly and caused the rift.
I think he rushed a new relationship and is now paying the price for moving too quickly. Did you meet via OLD?