I really feel like I have a dilemna on my hands. I don't want to tell H that S told me what he has been saying, but I feel it is important to tell him to leave the boys out of this and stop badmouthing me.
You do have dilemma on your hands.
I think this is a tough call, but if it's me, I would have to put the kids' interests above even my M. Your H isn't.