Ah, ok. Now I see. If you've got that level of verbal abuse and passive-aggressiveness, then you're right, it's kind of a no-win situation. do you think if you had S go to his house (or is he now? I thought you tried that before but can't remember....) and you weren't present, that things would be any better for S? What do the verbal abuse experts say about co-parenting visits, neutral 3rd party hand-offs, etc. or do they have any suggestions?
Is it a possibility to ask your L to file a motion with the court to have a child advocate/specialist come in to evaluate things? Now, of course H will be on best behavior then, but they may talk to S, too. And I know you have all sorts of issues then if S is scared of saying bad things about dad, but this isn't the first time they've seen this, so I'm sure they have ways of getting at the truth. Then either H would respond to this b/c he doesn't to anything else, or the court would help you create a safe plan for S in terms of visitation, etc. Is this an option?
I know L is your last option and you don't want to do that, but if MC isn't possible and co-parenting therapist person isn't either, I'm not sure what other choices you have left to you. And it sounds like things are just getting worse, so I am concerned about you acting sooner rather than later----
((((Hope))))
When the men on the chessboard Get up and tell you where to go; And you've just had some kind of mushroom And your mind is moving slow; Go ask Alice... I think she'll know.