You mentioned you D19, how old is your S? I ask b/c at 19 she is old enough to speak for herself. That doesn't mean you can't put an observation on the table. I wouldn't couch it terms of his being "careful" b/c then you're telling him what he can and can't say to his kids. My D's are 17 and 14 and I let them speak for themselves mostly. If I do need to say something, it is strictly "X shared this with me".
If your kids aren't good at speaking from the heart in a face to face, perhaps a letter or text would work for them.
