"Every madman thinks all other men mad"
Oh for sure, I'm the mad Men because I think children shouldn't be given surgery and life-altering hormones based on thoughts they have as children. Your position is so terrible that there's nothing you can say that will make it ever make any kind of sense.
But instead of arguing with him, getting angry with him, expressing sorrow for his kid, or even acknowledging him and agreeing to disagree about their parenting methods, you keep talking about your imaginary neighbor.
You are getting so wrapped up in your talking points that you're forgetting to tell a believable story or have believable reactions.
And no, I didn't respond to that guy. Because you're right, I do think he's doing/did a terrible thing. So instead of harping on him for something it's too late to even change, I let it go. Just like I do/did for my neighbor. I've helped that guy with his car twice over the last two years (tire rope plug, battery, small fluid checks), and didn't mention that evil stupid shit he did/allowed his kid to do or go through.
I rather argue this shit out with people that can change, not with the few specific people so deep you'll never change their mind.