It won't have come from one place. It's everywhere.
I'm a 40-odd year old bloke. I know that men cry, I do it on occasion myself (most recently was yesterday at an episode of Young Sheldon). I will happily tell people I cry. I am not ashamed of crying, whether its at something in real life like my Mums death, or something as stupid as a child being bullied on a sitcom.
And yet I still always try to hide it when it happens, because I don't like to people to see me doing it. It makes me feel weak, and shameful in the moment, even though I don't feel that way afterwards.
And it's because boys don't cry. Everyone knows that. It used to be that you'd get picked on for it, Dad would call you a wuss, friends would laugh at you. That has mostly flipped. You now get cartoons with the message that it's OK to cry, parents tell their boys that it's OK to cry, you get threads like this.
That's great, but it also reinforces the message that boys crying isn't something that should happen, but it's OK when it does. It's not normal behaviour.
A 16 year old kid seeing this thread (not sure how many of them are on Mumsnet admittedly) isn't getting the message that it's OK for boys to cry, they're seeing that a boy crying is an event that's worth making a thread about.
It's like going on holiday to Wales and seeing a sign on a cafe door saying "English People Welcome". You've not experienced any anti-english sentiment, everyones been lovely! Looks like the people in this cafe will be too. But there must be a reason for that sign to be there in the first place. It didn't appear in a vacuum.
The message at the moment is that boys crying is OK. The subtext of that message is still that it's abnormal. Once no-one comments on it at all, that'll be the point where boys really feel OK to cry.