Birthfamily ready to move on..Long b/c that's the only way I know how to talk

I know that the situation is normal and maybe even healthy but I am sad for my son.... 7 years ago we had a very open adoption through fostercare. G'parents are wonderfully caring people. The g'mother took the adoption hardest. After the adoption, she wanted to meet twice a month:o We went to extraordinary lengths to accommodate those requests. I really wanted to help her anyway I could to make peace with the adoption. This has slowly declined to 2-3 times a year. Fast forward 6 years. Her oldest daughter starts having children. She finally has "her own grandchildren" to dote on and maybe time has finally healed those wounds. I just called to set up the visit and I kinda' got the feeling that she was meeting us out of obligation. So now I find myself in this dilemma. She was pushing for all these visits but now seems more "emotionally" distant. I completely understand that this is normal and healthy for her but I'm worried how this will affect my son. He adores them. Mostly b/c he's young (7yo) we have told him none of the sordid details but that the adoption was a loving act by his mother. I don't even want to imagine how crushed he will be if his g'parents didn't want to see him anymore. I feel like I should have THE TALK. The dreaded conversation about whether she wants us to continue visiting. I'm so awkward with this kind of stuff. I don't know how to open the conversation..please, PLEASE, someone help me.