When my wife leftme and went to live in Germany with her new man, I naturally wanted to know where my children were living and also to meet her new partner ( although I had every faith in my wife that she would never make a negative decision regarding my children's welfare).
She acquiesed to this request and I went over to meet the new man.
I don't like im as an individual, but he appears to be essentially honest and decent and treats my children as if they were his own, therefore I don't HAVE to like him on a personal level.
Equally, my ex-wife likes to know who i am seeing and where the kids would stay if they came to see me.
It's normal and reasonable to want to know this on both sides, so I suggest that both your son and his ex wind their necks in and get on with behaving in a way that is best for their kids.
Relationships go wrong, parenting never should.