I agree that boundaries should be maintained but if he makes plans to take his son camping then problem solved? However, I get the "sneak attack info" from my partner like this too about his kids. He's afraid I'll get anxious and require soothing when he just wants time with is kids (who drink heavily and trigger my inner al anon but basically are not my business).
What helps me is to stay in my lane focus on my business and let his son be his business. It's a challenge but going to al anon meetings and staying busy with my own things help me to avoid the bad guy feelings and allow them to be HIS issue.
That being said, if one of these kids ever did or said any thing directly to me, in any way, I would be all over it. But for me, it's the perception that it's my business when it's not. As long as they are dealing with their father, I can be light and polite and carry on focusing on making my life better.
I can only say this after years of taking them on and giving unneeded input. When my son's drug issues became my focus, I had to learn boundaries with him which taught me to apply it to his kids.
Plus I learned this: (detach)
DON'T
EVEN
THINK
ABOUT
CHANGING
HIM/HER
(which also applies to my partner) I can only have courage to change myself. Good luck to you, God bless